CLIOTHYRIDINA 473 



completely removed from the fossil specimens, but usually some bases of 

 the spines or more complete fragments of the lamellae can be detected 

 unless the shell has been too badly exfoliated. This is the commoner 

 genus of the two in our faunas, and the shells are usually smaller and 

 more lenticular than Atliyris, although this statement is not applicable to 

 all the species of the genus. 



CLIOTHYRIDINA GLENPARKENSIS n. sp. 

 Plate LXXVIII, Figs. 21-24 



1909. Cleiothyris royssi Weller, Bull. Geol. Soc. Am., vol. 20, p. 313, 

 pi. 14, figs. 1-3. 



Description. Shell lenticular in form, the two valves nearly equally 

 convex, transversely subelliptical in outline, the length about six-sevenths 

 of the width, the greatest width near the mid-length of the shell, the 

 hinge-line much shorter than the greatest width. The dimensions of a 

 nearly perfect individual are : length 18 mm., width 21 mm., thickness 

 9.5 mm. 



Pedicle valve moderately convex, the greatest depth posterior to the 

 middle, the surface curving most abruptly to the cardinal margin; the 

 mesial portion of the valve slightly flattened, and sometimes slightly 

 depressed anteriorly in a narrow, obscure sinus; the beak small, pointed, 

 in close contact with the umbo of the opposite valve, pierced by a sub- 

 circular foramen ; cardinal area obsolete, the delthyrium broadly triangu- 

 lar and filled by the beak of the opposite valve. 



Brachial valve subequally or sometimes slightly more convex than the 

 pedicle, the greatest depth near or a little posterior to the middle, the 

 surface slightly compressed towards the cardinal extremities; the mesial 

 portion of the valve flattened along a narrow band from the beak to the 

 front margin, but not elevated into a, fold, the mesial flattening of the 

 valve being more conspicuous than that of the opposite valve; the heak 

 incurved beneath that of the opposite valve and filling the delthyrium. 



Surface of both valves marked by regular, fine, crowded, concentric, 

 imbricating, lamellose lines of growth, which, when the surface characters 

 are perfectly preserved are produced into rows of closely imbricating 

 fringes of flattened spines. 



Remarks. The species Atliyris roissyi has been given so broad an inter- 

 pretation by Davidson, 1 as to include essentially all of the species here 

 referred to the genus Cliothyridina, but as used by DeKoninck, the name 

 applies to shells which are essentially like those here described as C. glen- 

 parkensis; Leveille's original illustrations of the species, however, represent 

 a shell with a well-developed fold and sinus similar to the American C. 



1 Brit. Foss. Brach., vol. 2, p. 84, pi. 18, figs. 1-11. (1860.) 



