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ART. XXVII.- Structure and Appendages of Trinucleus ;
by CHARLES E. BEECHER. (With Plate III.)
TRINUCLEUS departs so widely from the common type of trilobite form, that any contribution of new facts regarding its structure and appendages is a matter of interest. Moreover, this added information will be of assistance in interpreting some peculiar and striking features in the natural group of genera of which Trinucleus is evidently a member.
For the present, it is convenient to consider in this group such forms as Trinucleus, Harpes, Harpides, Dionide, and Ampya. Most of these have the genal angles extending to or beyond the pygidium, with a broad, finely perforated or punctate margin around the head. They are further characterized by the absence or obsolescence of visual organs, while the facial sutures are either peripheral, as in Harpes, or in addition include the genal spines, as in Trinucleus, Dionide, and Ampyx. Several other genera have been recognized as having affinities with those mentioned, but they are imperfectly known, and will be merely noticed here. Tarpina, Novák, based upon the features of the hypostoma, is probably of only subgeneric value under Harpes. Arraphus, Angelin, is apparently based upon a specimen of Harpes denuded of the punctate border. Salteria of W. Thompson, and Endymionia of Billings, both generally considered as closely related to Dionide, were founded upon too imperfect material to afford decisive data as to their affinities. Angelin's sub-genera of Ampyx (Lonchodomus, Raphiophorus, and Ampy) are based upon the length of the glabellar spine, and the possession of five or six free thoracic segments. Similar characters in Trinucleus are not considered as worthy of such marked distinction,
In 1847, Salter* illustrated and described an eye-tubercle on each cheek of Trinucleus, from which there was a raised line extending obliquely upward to a punctum or spot on each side of the glabella. He considered this line as a discontinuous facial suture, but the true suture was afterwards correctly determined by Barrande,+ and in well-preserved specimens, may easily be observed, extending around the entire frontal and lateral border of the head, and including the genal spines. The "eye-line” was further recognized by McCoy, # and made one of the bases for a division of the genus into two sections or genera-Trinucleus proper and Tetraspis. These divisions were accepted by Salter, but later were thoroughly discussed, and rejected by Barrande (l. c., p. 617), upon valid grounds. Nicholson and Etheridge, * in 1879, reviewed these facts at some length, and gave original figures illustrating the ocular tubercle and eye-line. They also agree with Barrande in recognizing them as clearly adolescent characters.
* On the structure of Trinucleus, with Remarks on the Species, Quar. Jour. Geol. Soc., vol. iii. pp. 251-254. + Syst. Sil. Bohême, I., 1852. Ann. Mag. Nat. Hist., 2d Series, vol. iv., 1849.
The justice of these conclusions is substantiated, and additional results are reached, from the study of a series of Trinucleus concentricus Eaton, found associated with Triarthrus Becki Green, in the Utica slate, near Rome, New York. The remarkable preservation of the fossils at this locality, has already afforded a means of determining all the principal details of the ventral structure of the trilobite genus Triarthrus, and there is now distinct evidence as to the nature of the appendages in another type-Trinucleus, as well as to the probable significance of the so-called "eye-tubercle."
As compared with Triarthrus, specimens of Trinucleus are not very common at this locality, and, although more than fifty individuals of the latter have been obtained from the collections presented to the Yale Museum by Professor Marsh, not more than half a dozen of these are adult specimens, and but three show any appendages. Young specimens of all ages occur, from about 1mm across the cephalon upwards, and in all the eye-line and eye tubercle are present until a width of nearly 5mm is attained, when in the present species these features dwindle and disappear, leaving no discoverable traces in the adult..
Two cephala of young individuals, without the free cheeks, are shown enlarged in figures 1 and 2 of Plate III. Figure 2 represents a specimen before the appearance of the perforate border, and figure 1 gives a later stage, having two rows of perforations around the head. On both specimens the eveline is clearly shown, extending somewhat obliquely backward from the anterior lobe of the glabella to the central area of the fixed cheeks, enlarging slightly, and terminating in a rounded node or tubercle (a, a, figure 2).
In seeking for homologous features in other trilobites, the genera llarpes and Harpides are immediately suggested, since they have similar ocular ridges extending from the sides of the glabella, and ending in a tubercle, which, in Harpes, contains from one to three eye.spots, as determined by Barrande. They further agree in having these visual organs on the
* Monograph of the Silurian Fossils of the Girvan District in Ayrshire, Fasc. II., 1879.
fixed cheeks, while in all other trilobites with distinct eyes, the free cheeks carry the visual areas. This type of eye is thus quite different in its relations to the parts of the cephalon from that of Phacops or Asaphus, and more nearly resembles the eyes of some of the Merostomata (Bellinurus), as do also the triangular areas in the young Trinucleus, so distinctly marked off from the fixed cheeks on each side of the glabella behind the eye-line. Adult Trinucleus and llarpes have these areas much reduced, and often obsolescent. A spot or node in the median line on the glabella has been noticed by many observers, and although its nature has not been demonstrated, it has generally been called an ocellus. It is more clearly preserved in adult specimens, though it can be detected in young examples, as indicated in figures 1, 2, Plate III.
An eye-line occurs in many early trilobite genera, and is well marked in Conocoryphe, Olenus, Ptychoparia, and Arethusina. At least four-fifths of the Cambrian forms preserve this feature, which is almost entirely eliminated before Devonian time. It differs in extent, but not necessarily in nature, from the eye-line of Trinucleus and Harpes in running entirely across the fixed cheeks to the free cheeks, ending in the palpebral lobe in eyed forms. It is evidently a larval character in the trilobites, as shown from its geological history and the ontogeny of Trinucleus. From the direction of the optic nerve in Limulus, and its relations to the surface features of the cephalothorax, the eye-line probably represents the course of that nerve, and is of much less morphological importance than the different types and arrangement of visual organs.
The pygidium of young 7. concentricus (Plate III, figure 3) is remarkable for the lack of definition between the axis and pleura. In later and adult stages the number of ridges on the pleura and axis do not correspond, and from figures 4, 5, and 6, it is evident that in this genus the number of pleura is no indication of the number of pygidial segments or pairs of appendages, which, however, may be shown, as in this case, by the annulations of the axis. In this respect, the pygidia in Encrinurus, Cybele, and Dindymene, are of the same nature. Figure 6 also shows a narrow, striated doublure, a character generally overlooked in descriptions of Trinucleus.
Appendages. Three specimens have thus far been observed which show the nature of the appendages in Irinucleus. Two of these are illustrated in figures 4, 5, and 6, of Plate III. Figure 4 represents the thorax and pygidium viewed from the dorsal