162 TWENTIETH REPORT ON THE STATE CABINET. 



our appreciation of generic limitations will not be -enhanced by including 

 under the same term the P. lens and P. oblongus =z P. Icevis. 



Though at first restricting the genus to the Middle Silurian formations, 

 Mr. Billings has, at a later period, included under 8tricklandinia the 

 Pentamerus elongatus of Vanuxem = RensselcBria 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 N/- shaped 

 pit supported by a septum precisely as in Pentamerus; 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 associated with P. liratiis, 

 P. gaspe7isis, P. canadensis and P. anticostensis, which have a straight 

 hinge-line, an area on the ventral valve with a septum supporting a short 

 \/- shaped pit, a different muscular imprint and hinge-processes of the dor- 

 sal valve, and a fibrous or lamellose structure of shell, and are otherwise 

 externally marked in a very characteristic manner. 



I have taken some pains to bring together specimens of P. (R.) elongatus^ 

 and we have now the means of knowing its real characters to a very conside- 

 rable extent. As before observed, the dorsal valve presents all the charac- 

 teristics of Renssel^ria in its hinge-structure and muscular impressions : 

 there is the same narrow foramen beneath the hinge-plate, a pedicle-opening, 

 and the filling of this is preserved in several specimens of the casts; while 

 this portion of the separated valve shows the same features in a most une- 

 quivocal manner, and it is quite impossible to point out any characters by 

 which it may be separated from the same part of the shell in Renssel^eria. 

 The crura are free from near their origin, and have been traced for some 

 distance within the cavity, and gently curving to the ventral side ; but their 

 termination is unknown. When, however, we examine the hinge of the ven- 

 tral valve, we find a modification of the dental plates, which are prominent 

 in Renssel^ria (as shown in fig. 3 ^, A & m, Plate cviii. Palaeontology 

 of New- York, Vol. iii), but which are here produced anteriorly and united 

 at their dorsal margins. The incipient septum in the bottom of the valve of 

 Renssel^ria becomes developed, and sustains the united bases of the 

 dental plates ; and we have the \/- shaped pit and central septum of 

 Pentamerus. 



In the separated valves of this species, the margins of the triangular 

 fissure of the ventral valve are as clearly defined, and as free from area, as 

 are the valves of Athyris or Meristella. 



Generic significations must have some limit, and we cannot consent to 

 unite three so widely differing and well-marked types as P. liratus, P. gas- 

 pensis, etc. with P. oblongus and P. lens on the one hand, and P. elongatus 

 on the other. 



There is also another type usually included under the Genus Pentame- 

 rus, which may be separated with advantage. This one is represented by 

 P. reversus (Billings), P.verneuili and P.interplicatus (Hall), etc. In 

 these forms the relation of the valves, as in typical Pentamerus, is 

 reversed, and the interior structure presents other important differences. 

 For these I shall propose the name of Anastrophia. 



There is still a farther separation required among the pentameroid forms, 

 or an extension of the characters of the genus. The Pentamerus occidtmlai/s 



