GENERA OF THE SUBORDER PENTAMEROIDEA 



185 



cruralium being developed from the type with the 

 parallel plates by deposition of adventitious shell mat- 

 ter between the plates. Specimens from Mannie, 

 Tennessee, examined by us appear to conform to the 

 first type, that with the pseudocruralium, and it is 

 to this form that the name must be restricted unless 

 further acquisition of material shows that Platymerella 

 s. s. develops from a type with parallel plates and no 

 adventitious material. One specimen from Illinois sec- 

 tioned by us is of the Pentamerus type, and probably 

 the Alexandrian species from Illinois now referred to 

 P. mantuensis are actually not the same species and 

 may not be of the same genus. 



Foerste has suggested that Platymerella is closely 

 related to Pentamerella. The cardinalia are sugges- 

 tive of this relationship but the shape of the valves, the 

 nature of the beaks, and the structure of the spon- 

 dylium are quite different. Furthermore, there is 

 an enormous time discrepancy between the two genera 

 which makes the relationship rather remote. Platy- 

 merella is of great interest because it is the earliest 

 known pentamerid (together with the Pentam.erus-\\\it 

 form referred to above). Here again a plicated form 

 introduces a large group of shells. 



Genus VIRGIANA Twenhofel 1914 



PI. 27, figs. 1,6, 7, 16; t. fig. 35 

 Twenhofel, Geol. Surv. Canada, Mus. Bull. 3, 1914, p. 27. 



Genoholotype. — Pentamerus barrandei Billings 

 1857, Geol. Surv. Canada, Rept. Prog, for 1856, 

 p. 296. 



Description. Exterior. — Elongate-oval, hinge- 

 line short, lateral profile biconvex, the ventral valve 

 having the greater convexity. Anterior commissure 

 sulcate, uniplicate, or nearly rectimarginate. Ventral 

 valve usually with a low fold, which in young stages 

 forms a median plica in a low sulcus; the dorsal valve 

 in young shells usually has a noticeable fold, which 

 may reverse at maturity into a sulcus or may remain 

 as a low fold. Interareas obsolete, ventral beak in- 

 curved strongly, umbo swollen; dorsal beak curved 

 under the ventral. Surface unevenly multicostate ; 

 shell substance fibrous, impunctate. 



Ventral interior. — Delthyrial cavity deep ; spon- 

 dylium duplex long, free anteriorly; duplex septum 

 short. 



Dorsal interior. — Dorsal valve provided with two 

 short, stout septal plates which extend nearly vertically 

 and attach directly to the internal surface of the 

 valve. 



Geologic range. — Early Silurian of North 

 America. 



Species 



ClorinJa barrandei Billings 18 57 

 Pentamerus decussatus Whiteaves 1891 

 Virgiana anticostiensis Twenhofel 1928 

 V. major Savage 1916 

 V. mayvillensis Savage 1916 



Distinguishing characters. — Virgiana is most 

 readily identified by its strongly convex ventral valve 

 and relatively gently convex dorsal valve, elongate 

 spondylium, and short, discrete septal plates. 



Discussion. — In the ventral valve of young indi- 

 viduals of V. barrandei, for 2 or 3 millimeters there 

 is a shallow sulcus, but after this distance a low 

 median plica develops in the depression. As growth 

 continues, the sulcus becomes shallower and finally 

 disappears, but the median plica enlarges and continues 

 to the front margin as a low fold. In some varieties 

 the median plica consists of a bundle of low costs. 

 The fold at the front margin is very gentle and is 

 usually narrow. 



On the dorsal valve of young individuals of V . bar- 

 randei the young are provided with a low plica, in- 

 dented medially by a shallow stria. With continued 

 growth and bifurcation of the median plica, the fold is 

 lost in the ornamentation of the valve. In some speci- 

 mens the antero-median portion of the valve is de- 

 pressed into a very shallow sulcus. It is this phenom- 

 enon of the "reversion" of the fold and sulcus that 

 Twenhofel considered the most important generic 

 character of his genus. However, in the specimens 

 from Anticosti in the Peabody Museum there are many 

 in which the dorsal fold maintains its identity through- 

 out the length of the valve. This is especially true 

 of the small V. anticostiensis. Furthermore, some 

 specimens of V. mayvillensis preserve the fold from 

 near the beak to the front margin. 



More important in the diagnosis of the genus than 

 reversion of the fold and sulcus is the internal struc- 

 ture of the valves. The ventral shell has an exceed- 

 ingly long spondylium which is free for most of its 

 length. In the dorsal valve the cardinalia are remark- 

 able for their simplicity and shortness; in one indi- 

 vidual having a length of 36.5 mm. the septal plates 

 extend for only 5 mm. These plates arc pentameroid, 

 uniting directly with the valve. This structure differs 

 from that of Clorinda in having the plates nearly 

 parallel in horizontal section and in not possessing the 

 inner carin.t which project into the cavity bounded by 

 the plates. The structure therefore is not greatly 

 different from that of Conchidium and we are remov- 

 ing the genus to the subfamily Pentamerina: where it 

 properly belongs. 



Hall and Clarke""^ figure an interior of Pentamerus 

 barrandei showing a spondylium in both valves, that 



"Pal. N. Y., vol. 8, pt. 2, 1893, p. 243, t. fig. 174. 



