THE BKACHIOPOD SHELL. 313 



The system of muscle scars in Trematoholus (PI. LXXXIII) -is, so far as known, that of 

 Oholus, with a modification probably of the position of the scars of the pedicle muscles; of the 

 latter, unfortunately, nothing is known. Dearbornia (PI. LXXXII, figs. 7b and 7c) has the same 

 arrangement of the muscle scars as Trematoholus, but of the muscle scars of the other genera 

 descendant from the Trematoholus stock we know very little. The interiors of Siphonotreta 

 (PI. LXXXI, figs. 6c, 6d, and 6e) indicate an arrangement somewhat like that of Trematoholus. 

 YorTcia, Orhiciiloidea as known in the Cambrian, and PJiilTiedra show nothing of the muscle 

 scars. Schizamhon (PI. LXXXIV) appears to be related in its muscle scars to Trematoholus. 



The muscle scars of Acrothele, Acrotreta, and Acrotlvyj'a agree in having the posterolateral 

 scars united on each side in one large cardinal scar situated in the posterolateral region of the 

 valves. The central and anterolateral scars of the dorsal valve are variously situated (PL 

 LVIII, figs. 5f and 5h, and PL LX, fig. 1 1) in the central portion of the interior of the valve, 

 but not more so than in the dorsal valve of Oholus. In the ventral valve the outside and mid- 

 dle laterals and the central scars are more or less contracted, and drawn back toward the small 

 visceral area about the pedicle opening ; the individual scars have not been seen except in one 

 specimen of Acrotreta argenta (PL LXVII, fig. 5b). In this species the visceral area is unusually 

 large and the positions of the outside lateral and central scars is shown ; the anterolaterals are 

 not delimited. Another species of Acrotreta, A. ophirensis (PL LXXIV, fig. le), has an unu- 

 sually large visceral area, but, although the area of the muscle scars is defined, the individual 

 scars can not be seen. No reference has been made to the umbonal muscle scars, as they are so 

 rarely preserved. They are well developed and shown in Oholus and Lingulella, and it is 

 highly probable if not certain that some form of umbonal muscle existed in all of the inarticu- 

 late brachiopods. 



PBOTBEMATA. 



The muscles of the Protremata are limited to three sets: diductors, to open the valves; 

 adductors (centrals), to close the valves; and the pedicle muscles. 



In Kutorgina, the atrematous progenitor type of the Protremata, the anterior and poste- 

 rior adductors (equal to the anterolateral and central scars, respectively) are known in the dorsal 

 valve (PL V, fig. Ih), but nothing is known of the point of attachment of the diductor muscles 

 in the dorsal valve or of any muscles in the ventral valve. The muscle scars are preserved in 

 Billingsella in the ventral valve within the area of the pseudospondylium-. The diductors 

 probably occupied the outer divisions of the tripartite pseudospondylium and the adductors 

 the central section. This is well shown in many specimens (PL LXXXV, figs. Im and Ip; 

 PL LXXXVI, figs. 3g, 3i, 3j, 3k, 3m, and 4b; PL LXXXVII, figs. 5b, 5c, and 7; and PL XC, 

 figs. 2g, 2h, and 2i). The diductors appear to have followed closely in the line of the scars of 

 the main vascular sinuses if the tripartite pseudodeltidium is considered to have been wholly 

 taken up by the points of attachment of the muscles. In the dorsal valve the adductor scars 

 are often prominent (PL LXXXVIII, figs. Ig, Ih, Ij, and Ik), also the cardinal process to which 

 the diductor muscles were attached. The interiors of the ventral valves have not thus far 

 shown muscle scars. 



The muscle scars of the ventral valve of Nisusia and its subgenus Jamesella, and those of 

 Finkelnhurgia, Orusia, and Protorthis, were probably gathered on a pseudospondylium or, 

 when the latter is iiot defined, on the bottom of the valve beneath the umbo, as in Eoorthis. 

 In an obscure specimen of the dorsal valve of Portorthis quacoensis (PL XCIX, fig. 2d) the pos- 

 terior adductor scars appear to be preserved. 



In the dorsal valve of Fiiikelnhurgia the adductors are finely shown (PL XCIII,fig. If), also 

 a rounded, posterior central depression in which the diductors may have been attached. 



Among the Cambrian Syntrophiidje muscle scars have been discovered in Huenella ahnor- 

 mis of the Upper Cambrian. In the ventral valve (PL CHI, fig. 2b) the adductor and diductor 

 scars are preserved on the spondylium, and dorsal valves (PL CIII, figs. 2f, 2i, and 2j) show 

 the adductors in fine preservation, also the cruralium to which tlae diductor muscles were 

 attached. The muscle scars of this species are of unusual interest, as the ventral valve has a 

 spondylium to which the pedicle, the large diductor, and the smaller scars of the adductor 

 muscles were attached. 



