﻿ARKANSIA 477 



Arcidens confragosus Simpson, Syn., 1900. P- 661.— Ortmann, 

 Ann Car. Mus., VIII, 1912, P- 284, figs. 12-120. 

 A remarkable form, which does not seem to be very closely 

 related to any other known species, except the recently de- 

 scribed Arkansia zvhcelcn. The sculpture is exceedingly com- 

 plicated, that of the beaks with the two rows of nodules being 

 something as it is in Qitadnila lachryinosa. The oblique wavy 

 folds recall those of the plicata group of Onadnda, but the 

 teeth do not show any affinity to^ this group. The radial wrin- 

 kles and sulcations differ from those of any species I know of. 

 In the only gravid specimen I have seen the marsupium pre- 

 sented a peculiar striated, granular structure, different from 

 that of any Naiad I know. It was narrowed in the middle, 

 and produced into a sort of lobe behind, but, though the spec- 

 imen seemed to be a perfectly normal one, I am in doubt wheth- 

 er this peculiar form is constant. I do not think that this 

 granulation is the result of a diseased condition. 



Genus ARKANvSIA Ortmann and Walker, 191 2. 



Arkansia Ortmann and WAi^KiiR, Naut. XX\', 1912, p. 97. 



"Shell moderately thick, subrotund to subovate or subrhom- 

 boidal, inflated, with full beaks. Disk sculptured with irreg- 

 ular, oblique folds, which are sometimes indistinct. Beak 

 sculpture poorly developed, consisting of two to three double- 

 looped bars, the loops slightly swollen or tubercular, disappear- 

 ing toward the disk and not continuous with the sculpture of 

 the latter. Hinge well developed, with strong pseudocardinals, 

 a very strong interdental projection in the left valve and well 

 developed, strong, but rather short laterals. 



Soft parts : Supra-anal opening separated from the anal by a 

 mantle connection, which is somewhat variable, but rather long 

 (a little shorter to somewhat longer than the anal; the supra- 

 anal is correspondingly longer or shorter ; in one case there is 

 a double mantle connection). Mantle edges, between the anal 

 and branchial, drawn together by the gill-diaphragm but not 

 united. Inner edge of anal almost smooth, that of the branchial 

 with papillc-e. 



