710 GEOLOGICAL SURVEY OF IOWA. 



in outline, often subalate. Dorsal valve with a rounded or 

 subangular mesial fold which expands rapidly towards the 

 margin of the shell, defined at the sides, simple at its origin, 

 bifurcating just below the apex, and each division dichoto- 

 mizing once or twice before reaching the base of the shell. 

 Ventral valve a little more convex than the opposite ; mesial 

 sinus varying from shallow and curvilinear to deep and sub- 

 angular, simple at its origin, the bordering plications bi- 

 furcating just below the beak, and one division on each side 

 going to form the bottom of the sinus ; the plications con- 

 tinue to dichotomise mainly on the sides of the sinus, until 

 there are three, four or five plications on each side of the 

 centre : beak somewhat abruptly arched over the foramen, 

 which is somewhat wider than high; area with the sides 

 nearly parallel, extending to the cardinal extremities, con- 

 cave in the middle and flat on each side, strongly striated 

 vertically. 



SURFACE of the shell marked by rounded or subangular 

 plications, which are disposed in fasciculi : plications crossed 

 by fine concentric striaB. 



. The form of this shell varies, in a considerable degree, in specimens from different 

 localities. It is in general somewhat inflated, and ranges from trigonal to semicircular, 

 depending on the prominence of the lower margin of the shell. The umbo is often 

 very gibbous, and the beak abruptly incurved; but in this character there are some 

 variations. The mesial sinus is subangular above, and becomes broader and gently 

 curved towards the base, rarely subangular throughout. The plications are disposed 

 in fascicles, and, in some specimens, nearly uniformly of three plications in each : 

 others are irregular. The mesial elevation is often marked by two fascicles of three 

 plications each, even when the remaining plications are irregularly disposed. 



A comparison of specimens from different localities shows that the one described 

 by me in Captain STANSBORY'S Report, under the name of Spirifer triplicatus, is 

 identical with specimens from New-Mexico, which correspond with the figures given 

 by ROOMER, of S. meusebachanus. Specimens from a limestone in the Coal measures of 

 Greentown (Ohio) are more extended laterally, and more nearly semicircular than 

 the southwestern specimens, and likewise present some slight differences in the 

 strength and disposition of the plications; but they are clearly identical species. 



A farther comparison of the original specimens of the species in Dr. HILDKETH'S 

 collection, and numerous others from Illinois and Iowa, shows no constant difference 

 in the forms, though the degree of variation is sometimes excessive. It is not im- 



