GENER.\ OF THE SUBORDER PENTAMEROIDEA 



163 



Siehrrella — smooth and costate stocks are essentially 

 contemporaneous, although in America the Harrand- 

 ellas and Clorindas evidently appear first, in the Brass- 

 field formation. With the Pcntamcrina; the first to 

 appear is Plotymerella (Brassfield-Alexandrian). It 

 may be the forerunner of Pentamerus, since the latter 

 nearly always shows broad radial costs, but whether 

 these represent secondary dying out of stronger costa- 

 tion or incipient development of the same is not known. 

 Conch'uiium was evidently always costate but may 

 have given rise to two secondarily smooth genera. 



3. Interareas. — It has been the common belief 

 among paleontologists that the pentamerids have lost 

 their interareas. This is certainly in part true. The 

 reduction of the interareas, and their subsequent loss 

 in a few stocks, are consequent upon the narrowing of 

 the hinge-line and the development of rostration. It is 

 very possible that this rostration has gone hand-in-hand 



Fig. 25. — Conchidium biloculare (Linnseus), from Klin- 

 tehamn, Gotland. Section showing unusual deltidial cover 

 of ventral valve. See pi. 29, fig. 4. The whole ventral 

 interior, except for the two roughly oval areas on either 

 side of the spondylium, is filled with adventitious shell, 

 represented by the stippled portions. 



with the elongation and narrowing of the spondylium 

 in the Pentamerinae in order to produce more surface 

 for attachment of the diductor muscles. In the 

 Gypidulinae, where the spondylium has not been so 

 pronouncedly narrowed, there are usually preserved 

 well marked remnants of the interareas. This is par- 

 ticularly true of Sieberella. 



Another feature of pentamerid shells in the vicinity 

 of the beaks is the production of prominent flattened 

 areas which greatly simulate an interarea. This fea- 

 ture is perhaps most strongly developed in Brooksina 

 which has practically no interarea but has a broad flat 

 region that extends from the beaks of both valves to 

 the point of contact of the ribbed portions. These 

 false cardinal areas are best developed in that genus and 

 in Capellinirlla, both groups having the convexity of 

 the valves reversed from the normal, and there may 

 be some mechanical connection between the two 

 phenomena. 



Ventral interior. — Important features of the 

 ventral valve are the delthyrium and its accessories, the 



deltidial cover and deltitidal plates, and the spondylium 

 and its supporting septum. 



/. Delthyrium. — So far as our observations have 

 extended, the delthyrium of the Camerellida: is open 

 except for marginal thickenings along the dclthyrial 

 border, alluded to below. No deltidial cover such as 

 occurs in Conchidium has been seen. 



2. Deltidial cover. — In Conchidium there is a truly 

 remarkable cover to the delthyrium which we hesitate 

 to call a deltidium, preferring the more non-committal 

 term deltidial cover. As commonly described, this has 

 been termed a "concave deltidium." Our investiga- 

 tions show that this structure is concave toward the 

 anterior portion of the delthyrium, but when followed 

 to the vicinity of the beak it rises above the margins 

 of the delthyrium and projects dorsally in the form of 

 a sheath with rather rectangular sides and depressed 

 center. A similar structure exists in one specimen of 

 Harpdium, but was erroneously described as convex 

 deltidial plates. These two occurrences are the only 

 ones noted by us in which there is a cover plate over 

 the delthyrium. A concave "deltidium," presumably 

 like the above, has been reported in Pentamerus but 

 we have not seen such. The structure in Conchidium 

 appears to be a pedicle sheath ; it is not known definitely 

 whether it had an open foramen at the apex, but it 

 appears to have had one. In the specimen of Harpid- 

 ium preserving the deltidial cover the beak is crushed 

 down on the cover so that its precise structure can not 

 be determined. Parenthetically it may be added that 

 among the orthids and strophomenids no such cover 

 as this has been observed. 



3. Deltidial plates. — Hall and Clarke announced 

 the presence of deltidial plates in several genera studied 

 by them and even went so far as to use these struc- 

 tures as one of the generic distinctions between 

 Gypidula and Sieberella. The present writers have 

 not observed in any of the pentamerids that have come 

 under their observation any clearly developed, typical 

 deltidial plates. We have seen thickenings along the 

 delthyrial margins of many of the genera, and such 

 structures as these were termed "pscudo-deltidial 

 plates" by Booker."' These thickenings, in our ex- 

 perience, never restrict the delthyrium in any notable 

 degree, certainly not nearly so noticeably as do similar 

 thickenings in Glossorthis, Hesperorthis, and other 

 orthid genera. Similar structures were observed also 

 in specimens of Conchidium that have the deltidial 

 cover as well. It therefore appears to us that they are 

 of little taxonomic significance, certainly not sufficiently 

 important to warrant the removal of the Pentameracea 

 (restricted) to the Telotremata, as Kozlowski has 

 done. 



' Booker, Jour. Proc. Roy. Soc. N. S. Wales, vol. 60, 

 1927, p. 134. 



