INVERTEBRATES. 321 



This little shell resembles P. muricatus, of Norwood and Pratten, hut is 

 smaller, and has a more convex ventral valve. It also differs from all the spe- 

 cimens of that species we have seen, in the peculiar sudden bifurcation of its 

 costae on a somewhat raised band around the free border of its ventral valve. 



Mr. Thomas Davidson, of London, the highest authority on the fossil 

 firachiopoda, after examining specimens of P. wabashensis, P. splendens, and 

 P. muricatus (N. and P.), from Illinois, pronounces them almost certainly all 

 identical with P. longispinus, of Sowerby; and since we have seen his figures 

 of the varieties of that species in his Monographs of the Carboniferous Brachio- 

 poda of England and Scotland, we are inclined to adopt his opinion. 

 Although none of these varieties agree with the little shell under consideration, 

 we should not be surprised if more extensive collections would bring to light 

 intermediate gradations connecting it with P. longispinus, Sowerby. 



Our figures 4a, 4& and 4c, of this species, show the divisions of the costse 

 around the front too fine, and crowded; the enlargement, figure 4^, gives a 

 correct idea of their appearance under a magnifier. 



Locality and position : Jefferson county, Iowa; Lower Coal Measures. 



Genus SYNTRIELASMA, M. and W. 



Synon. — Choristites (sp.), Fischer, 1825. Programme sur la Choristites. 



Terebratula (sp.), cI'Orbigny, 1842. Voyage dans Amer. Merid., vol. 3, p. 



42; (not Miller and others). 

 Spirifer (sp.), Hall, 1852. Stansbury's Report Exp, to Great Salt Lake, p. 



409 ; (not Sowerby). 

 Orthis (sp.), Salter, 1861. Jour. Geol. Soc., London, vol. xvii, p. 64; (not 



Dalman). 

 Syntrielasma, Meek and Worthen, Dec, 1865. Proceed. Acad. Nat. Sci., 



Philad., p. 277. 



Shell thin, gibbous or subglobose, in adult specimens; valves 

 articulated by teeth and sockets, in the typical species nearly 

 equally convex; hinge line straight and very short; area small, 

 partly common to both valves, but higher in the ventral valve, 

 where it is divided by a triangular, open foramen; beaks 

 incurved, subequal. Surface radiately striate, and ornamented 

 with large, radiating plications, which form deeply interlocking, 

 angular projections at their terminations in the margins of the 

 front — one of the plications a little larger than the others on the 

 middle of the dorsal valve, so as to assume somewhat the char- 



41 Sept. 21, 1S66. 



