182 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



account is given of the occurrence and structure of the 

 "concave deltidium" of C onch'tdtum . This plate in 

 most specimens is truly concave and bears a median 

 groove giving it the appearance of having been formed 

 by the symphysis of two plates. One specimen of 

 C. biloculare, however, shows toward the end of the 

 beak that the plate is elevated above the margins of the 

 delthyrium, forming a cover with nearly rectangular 

 sides. In another specimen of the same species the 

 plate is elevated fully 3 mm. dorsally from the del- 



specimens is more than half the length of the ventral 

 shell and in some it extends for nearly the full length. 

 The cardinalia, are, as is usual in the Pentameridae, 

 the most interesting and significant structures of the 

 genus. Here, as in Gyfidula, Pentamerella, etc., three 

 distinct "plates" can be determined. The inner crural 

 plates are convex and unite with a narrow flat shelf 

 forming the inner postero-lateral margins of the valve. 

 The union is marked by a low ridge and groove which 

 forms the suture line between the valve and plate. 



^ CD ci) (2) 



^^ 



Fig. 33. — Conchidium biloculare Linn. Sections 1-3 show the delthyrial plate in place at a considerable distance 

 (4.2-6.1 mm.) from the beak. It is here rather concave, having been depressed from an elevated structure like that in 

 fig. 25. In section 4 lateral plates are visible in the dorsal valve. Similar plates were seen in Strkklandia (see pi. 29, fig. 6) 

 but their function and meaning are unknown. Distance from beak: 



1 — 4.2 mm. 



2—5.0 



3—6.1 



4 — 7.4 



5—8.1 



6—8.7 



7 — 9.9 mm. 



8—11.0 



9—12.1 

 10—14.4 

 11—15.0 



thyrial margin, having nearly rectangular sides, and 

 bears a shallow median groove. There is no visible 

 evidence that it has developed by the symphysis of two 

 plates. Further, the "deltidial plates" are visible along 

 the delthyrial border in specimens carrying the cover 

 plate. This modified deltidium must therefore have 

 been a sort of pedicle sheath housing the proximal 

 portion of the pedicle. 



Conchidium is usually defined as lacking a "cardinal 

 area" (interarea), but in all of the specimens exam- 

 ined a narrow but very definite interarea could be seen. 



The spondylium duplex in Conchidium is remark- 

 able for the great depth of the delthyrial cavity and 

 the great length of the septum. The latter in most 



The inner plates thus form narrow lateral chambers. 

 Posteriorly the terminations of the chambers are thick- 

 ened, and together with a boss or callus beneath the 

 beak form the "cardinal process" or seat of diductor 

 attachment. The outer plates appear to be represented 

 by narrow longitudinal ridges, the base of the brachial 

 process. On the inner sides of some specimens, each 

 of these ridges bears a plate directed dorso-medially. 

 These are probably posterior extensions of the brachial 

 process which extend anteriorly with a slight curve for 

 a considerable distance ; they are curved very slightly 

 laterally but none were observed to unite into a "loop." 

 The two inner plates and bases of the brachial processes 

 are supported by thin divergent septal plates. 



