GENERA PENTAMERUS, &c. 373 



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 ocddentalis of the Iowa 

 Geological Report presents externally a depressed dorsal valve, with a median 

 fold on the lower valve ; while there is a distinct area bordering the fissure, and 

 this area is vertically striated as in many of the Spirifers. The ventral valve has 

 the trough-like pit, formed by the junction of the lamellae, greatly extended and 

 extremely incurved, and the dorsal lamellae or crura are divergent and present 

 some peculiarities. 



It is doubtless unsafe to base a distinction of genera upon external characters 

 alone, and even with a partial knowledge of the interior structure, we may be 

 misled ; but this group of shells presents itself to us under an aspect that will 

 admit of the following arrangement : 



1. Pentamerus proper, having rotund or gibbous forms, with the ventral valve prominent 

 in the middle, and the dorsal valve flattened or depressed towards the front ; lamellae of 

 the dorsal valve distinct : P. knightii, P. galeatus, P. pseudogaleatus. 



2. Elongate forms with the valves subequally convex, lobed or Bubsinuate ; internal struc- 

 ture essentially as in P. knightii, and of which P. obhmgus, P. leTis ? are typical forms. 



3. Forms ovate, more or less rotund, with a sinus on the ventral valve and a mesial fold 

 on the dorsal valve ; internal structure of the ventral valve as in P. knightii. Dorsal 

 valve with the crura or lamellae of the hinge-plate conjoined so as to form a separate 

 trough-shaped cavity, which unites with the inner surface of the valve ; a narrow area 

 on each side of the fissure, and a flattened space or false area along the cardinal margin 

 of the valve. P. aratiis and P. papilionensis are of this type. Genus Pentameeella. 



4. Forms more or less elongate, lobed or with mesial fold a'td sinus ; hinge with an exten- 

 ded area on the ventral valve : internally a short \/- shaped pit in the ventral valve, 

 supported by a septum. In the dorsal valve, the crura are free almost or quite from 

 their origin (as in Spikifera), and forming no vertical lamellae. P. liratiis and P.micro- 

 camerus are European forms of this type = the Genus STaiCKLANDiNiA of Billings ; 

 of which S. canadensis, S. brevis, S. gaspensis and S. anticostensis are characteristic forms. 



5. Short gibbous or ventricose forms ; the ventral valve much the larger, with or without 

 mesial fold, a large fissure, and elongate much incurved trough-shaped pit. Dorsal valve 

 depressed in front : an area on both valves ; that of the ventral valve striate as in 

 Spikifera : lamellae of dorsal valve separate and diverging. Genus Gtpidula, of 

 which P. ocddentalis, P. letviusculus and P. obsolescens are types. 



6. Rotund or gibbous forms, with the valves, as in ordinary Pentamerus, reversed. The 

 ventral valve is tbe smaller, gibbous in its upper part, depressed or sinuate below, with 

 the \/- shaped pit sessile for nearly its entire length ; a small flattened space on each 

 side of the fissure. The dorsal valve is ventricose, larger than the ventral, with promi- 



