THE BRACHIOPOD SHELL. 311 



is that of the Lower Cambrian Nisusiafestinata (PI. C, fig. Ij), wliere the plates form the support 

 of the teeth and also outhne a rudimentary spondylium. Protorthis of the Middle Cambrian 

 has a spondylium on each side of which are the teeth supported at each of the outer angles next 

 to the cardinal area (PI. XCIX, fig. 3b). Dental plates undoubtedly existed in other genera 

 of the Protremata of the Cambrian, but they have not been observed. 



VASCULAR MARKINGS. 



The most prominent of the vascular impressions are the pallial sinuses or main vascular 

 canals. Usually all that is impressed on the inner surface of the valves are the two main trunks 

 that diverge from near the apex or beak of the valves and skirt the margin of the visceral area. 

 In rare instances the impressions of the secondary branches of the main sinuses are preserved. 



ATREMATA. 



None of the shells of the genera of the Paterinidfe show more than the bases of the main 

 tnmks of the vascular system, but in Obolus and LinguleUa and their subgenera there are numer- 

 ous illustrations of the main vascular canals and in a few instances of the secondary canals. 



Oholus apollinis (PI. VII) of the Upper Cambrian has the vascular system as well developed, 

 judging from the impressions left by it on the interior of the valves, as in the mantle of recent 

 Lingula. In the closely allied form, LinguleUa granvillensis (PI. XXII, figs, lb and Id), from 

 the Lower Cambrian, there is every reason to thmk that the vascular system has reached its full 

 development. The main vascular trunks, lateral canals, and peripheral canals are all indicated 

 in the ventral valve (PI. XXII, fig. Id). Striking illustrations of the preservation of the mark- 

 ings of the vascular system are shown in many of the illustrations. (See PI. VIII, fig. Id ; PI. XII, 

 figs. Ic, le,9, and9a; PI. XIII, fig. Im; PL XV, fig. 11; PL XVII, figs. Ih and Ij; PL XXXVI, 

 figs. Ic and Id; PL XXXVII, fig. Ic; and PL XXXVIII, figs. 2b and 2d.) 



In Bicia (PL L), one of the oldest of the Obohdse, the main vascular trunks are very strong, 

 and they are prominent in Dicellomus (PL LII, fig^. Id and Ij ; PL LIII, figs. Id, 2, and 4a). 



NEOTB.EMATA. 



In the Neotremata, Oholella, the most primitive form, has the vascular trunks developed 

 (PL LIV, figs. 2g-n) on a scale comparable with those of Obolus, but, as is usually the case 

 among Cambrian bracliiopods, the calcareous shells do not appear to bear the impressions of the 

 secondary canals as well as do the chitinous or phosjshatic shells. . The main vascular canals of 

 Botsfordia (PL LVII, figs. 4g-m; PL LIX), Linnarssonella (PL LXXIX, figs. Im-p, 2, and 2a), 

 Trematobolus (PL LXXXIII), Yorhia (PL LXXXII, figs. Ih and 2), Schizamhon (PL LXXXIV, 

 figs, la, Ic, 2d, 3b, and 3c), and Dearhornia (PL LXXXII, figs. 7b and 7c) are all strongly devel- 

 oped. In Acrothele are found some of the most beautiful examples of the jDreservation of a highly 

 developed system of vascular canals (PL LVI, figs, la-d, 3b, and 3c; PL LXI, figs. Ic and Id). 

 Acrotreta has relatively large vascular trunks (PL LXIV, figs, li, Ij, and 2c; PL LXV, figs. If, 

 Ig, 3c, and 6; PL LXXIII, figs. 4c and 6), but no traces have been seen of the lateral canals. 

 The main trunks of Acrotliyra are also large (PL LXXVI, figs. 4a and 4b). Discinopsis (PL 

 LXXXII, figs. 5a-b) has two narrow, long, main trunks that originate back of the line of the 

 opening of the pedicle tube. 



PROTREMATA. 



In BillingseTla the great size of the main vascular trunks (Pis. LXXXV-XC) is character- 

 istic of the genus. The manner in which the anterior branches of the main trunks pass outward 

 toward the border is finely shown in specimens of B. plicatella (PL LXXXVI, fig. 3i). The 

 dorsal valve of B. exporrecta (PL LXXXVIII, fig. Ik) has two strong apparently main sinuses 

 on each side; the inner pair, however, may not be casts of the vascular sinuses, but may indicate 

 grooves made by the path of advance of the attachments of the posterior adductor muscles. 



The vascular system of Nisusia (PL C) and its subgenus Jamesella (PL CI), Protorthis 

 (PL XCIX), and usually of Eoorthis, has left no traces of its presence on the shell. On some 



