NOTICE OF PALEONTOLOGY OF NEW-YORK, VOLUME IV. 163 



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 bor- 

 dering 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 

 lamellge 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 struc- 

 ture, 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; lamellge of the dorsal valve distinct : P.knightiiy P.galeatus, 

 P. pseudogaleatus. 



2. Elongate forms with the valves subequally convex, lobed or subsinuate ; inter- 



nal structure essentially as in P. knightii, 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 PexNTamerella. 



4. Forms more or less elongate, lobed or with mesial fold and sinus ; hinge with 



an extended 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. niicrocamerus are European forms of this 

 type = the Genus Stricklandinia of Billings ; of which S. canadensis, 

 S. hrevis, 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 sepa- 

 rate and diverging. Genus Gtpidula, of which G. (P.) occidentalis and 

 G. IfEviusculus 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 N/'Shaped pit sessile for nearly its 

 entire length ; a small flattened space on each side of the fissure. The dor- 

 sal valve is ventricose, larger than the ventral, with prominent umbo. The 

 hinge-plate is extended in gradually converging vertical lamellas which are 

 joined to the shell throughout their length, while the crura are extended 

 into the cavity in thin free lamellae. Genus Anastropiiia*, of which Pen- 

 tamerus verneuilii, P.interplicatusnTidP.reversus are t3^pes. 



7. Forms elongate, not lobed. V^entral valve with connected dental lamellge, 



forming a trough supported on a septum. Dorsal valve with free crura : no 

 area. Shell-structure punctate. Genus Amphigenia :. Type Pentamerus 

 elongatus (Vanuxem) =^ Amphigenia elongata. 



These modifications of a type, of which, until recently, but two genera 

 have been recognized, are well marked in nature, and they seem to me to 

 demand some farther recognition than that of subordination to the Family 

 Rhynchonelhd^. I shall therefore propose the Family PENTAMERiDiB 



* I am aware that Mr. Shaler, of the Museum of Comparative Zoology at Cambridge, in a Bulle- 

 tin of that Institution, has proposed the name Brachymerus for these forms; but since that name is 

 preoccupied for a genus of Coleoptera, it cannot be adopted in this relation ; and my own determi- 

 nation of the generic distinction of these forms having been made long since, I shall adopt the name 

 proposed above. 



