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 occidentals 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 lame life, greatly extended and 
extremely incurved, and the dorsal lamellfe 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 subsinuate ; internal struc¬ 
ture essentially as in P. Icnightii, and of which P. oblongus, P. lens ? 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. aratus and P. papilionensis are of this type. Genus Pentamerella. 
4. Forms more or less elongate, lobed or with mesial fold and 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 Spirifera), and forming no vertical lamellae. P. liratus and P.micro- 
camerus are European forms of this type = the Genus Stricklandinia 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 
Spirifera : lamellae of dorsal valve separate and diverging. Genus Gypidula, of 
which P. occidentalis, P. Iceviusculus and P. obsolescens are types. 
6. Rotund or gibbous forms, with the valves, as in ordinary Pentamerus, reversed. The 
ventral valve is the smaller, gibbous in its upper part, depressed or sinuate below, with 
the \J- 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- 
