24 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



In endopunctate orthid shells modifications of either the delthyrium or notothyrium are of 

 uncommon occurrence. Only one instance of a true deltidium is known and that is in Kayserella, 

 which stands unplaced in the Orthacea. A chilidium is also known in Heterorthis (see pi. 20, fig. 

 19). It is strongly convex and arches over the carinate postero-dorsal portion of the carinate median 

 lobe of the cardinal process. The only suggestion we can offer in regard to its function is that it serves 

 for protection of the muscle attachments on the cardinal process. 



Of unusual form and interest is the umbrelliform chilidium of Productorthis (see pi. 4, figs. 

 15, 16). This plate has its origin in the growth of two discrete plates along the notothyrial margins. 

 These grow dorsally to the dorsal surface of the cardinal process, uniting and expanding into a cir- 

 cular plate that hangs over the distal end of the process like a small umbrella. Here again the 

 function may be one of protection for the muscle attachments. 



In some orthids there occurs an incomplete chilidium or what may be termed chilidial plates. 

 These are low plates projecting posteriorly from the notothyrial margins and forming low walls on 

 each side of the notothyrium. These chilidial plates are commonly thickened by callus deposit on 

 the interior and cemented firmly by the spread of callus covering all points of contact. Chilidial 

 plates characterize Nicolella, Deltatreta, Pomatotrema, and Planidorsa. 



From the above, it appears that the sporadic reappearance of the chilidium and chilidial plates 

 in late forms cannot have a direct genetic linkage with the older genera wherein these structures are 

 of continuous development 5 the former suggest, rather, atavistic returns of primitive features. In 

 the Clitambonitidse, where the normal genera possess a chilidium, its persistent absence in other forms 

 must mean a progressive change of considerable significance in establishing species and genera, for 

 example in A-pofnatella. 



MORPHOLOGY OF THE VENTRAL INTERIOR 



ARTICULATION 



Genuine articulation is well developed in all of the Orthacea of the Cambrian, and its origin is 

 therefore to be sought in the most primitive order, Atremata. 



In the orders Atremata and Neotremata the two valves are held in apposition by muscles, but 

 in the Protremata and Telotremata they are held in place by teeth and sockets and sometimes by 

 other articular devices that together act as hinges. In many forms the brachial process or brachio- 

 phore also assists in locking the valves, since it articulates with the tooth of the ventral valve and 

 helps to prevent lateral motion. 



It will be shown directly that the tooth of the ventral valve in Orthacea bears a socket on its 

 posterior surface and a fossette on its inner and dorsal face. The accessory socket of the posterior 

 surface receives a small denticle on the outside of the socket of the ventral valve and the tooth fits 

 into the dorsal socket. On the other hand, it will be shown in the discussion of sockets that the 

 crural fossette receives the carinate or expanded posterior edge of the brachiophore. Accordingly, 

 we see that the brachiophore is as important in the articulation of these brachiopod shells as the tooth 



^n.Q socket. 



Teeth. — ^Teeth and dental plates are unknown in the inarticulate orders Atremata and Neotre- 

 mata, but ventral teeth with their dorsal sockets are present in all the Cambrian genera of the 

 Orthacea, though they appear to be very rudimentary in Kutorgina, thought to be the most primitive 

 genus of the articulate order Protremata. On the other hand, dental plates are not always present in 

 the older Cambrian genera (absent in Nisusia), but later on they make their appearance and are then 



usually present.** , r , • c u a } 



The teeth or articulating apophyses are situated immediately laterad of the margins ot the del- 

 thyrium, or, in a few genera, at the intersection of the delthyrial margin and the hinge-line. When 

 viewed from the posterior, the teeth are roughly triangular in outline, the apex of the triangle being 



"sWalcott, Camb. Brach., 1912, p. 310. 



