GENEKA PENTAMERUS, &c. 371 



continued as far as the middle of the length of the valve, where their 

 extremities apparently become free. In these features, there is no essen- 

 tial difference between P. oblongus and P. knightii or P. galeatus. 



In Pentamerus lens, or a closely allied form, from Anticosti, the dorsal 

 valve possesses similar characters, and there is no area on the ventral 

 valve. Restricting the designation to such forms as P. liratus and P. micro- 

 camerus among the European species, there is good ground for the sepa- 

 ration ; but the other species do not appear to me congeneric. Through 

 the kindness of Sir W. E. Logan, I have been permitted to examine 

 the American species of Stricklandinia, S. gaspensis, S. canadensis and 

 S. anticostensis (Billings), and also the species referred to the European 

 P. lens. The first two appear to me to be congeneric with P. liratus, and 

 do possess the characters of the genus as described. Limiting, therefore, 

 the application of the term as here indicated, I think we have a well- 

 marked genus, which, typified by the species above named, may also 

 include others heretofore referred with doubt to Spirifera. I conceive, 

 however, that our appreciation of generic limitations will not be 

 enhanced by including under the same term the P. lens and P. oblongus 

 = P. lavis. 



Though at first restricting the genus to the Middle Silurian forma- 

 tions, Mr. Billings has, at a later period, included under Strickladinia 

 the Pentamerus elongatus of Vanuxem = Rensselaria elongata (Hall), a 

 Devonian species. This form was first placed under Pentamerus, from its 

 resemblance to P. oblongus ; and I finally referred it to Renssel^ria, 

 from the generally similar shape, similar muscular impressions of 

 the dorsal valve, and the prismatic or punctate texture of the shell ; 

 having at that time an imperfect knowledge of the interior structure of 

 the ventral valve. Now this species has no area on the ventral valve ; 

 but it has a V-shaped pit supported by a septum precisely as in Penta- 

 merus ; while both the muscular impression of the dorsal valve, and the 

 structure of the hinge-plate, are precisely the same as in Renssel^ria. 

 I cannot see, therefore, upon what good ground this shell should be asso- 

 ciated with P. liratus, P. gaspensis, P. canadensis and P. anticostensis, which 

 have a straight hinge-line, an area on the ventral valve with a septum 



