BRACHIOPODA. 237 



term, is the Pentamerus lavis, a shell with a smooth exterior, and of which Mr. 

 Davidson remarks : * 



" It is admitted now by palajontologists that P. Icevis, Sow., is the young of 

 P. ohlongus; and if it were necessary to strictly adhere to rules of priority, 

 James Sowerby's name, published in August, 181.3, would perhaps require 

 [have] to be adopted in preference to that of oblongus, given to the adult shell 

 by Mr. J. de C. Sowerby in 1839; but, when we read over Mr. James Sowerby's 

 unsatisfactory description, and examine his small, very incomplete figure, it 

 seems preferable to preserve for this shell the now generally adopted and well- 

 known designation of oblongtis." 



Pentamerus oblongus is a species of very variable contour, with a smooth 

 exterior, sometimes bearing a few broad and obscure radiating undulations, 

 transverse or elongate-oval in outline ; the valves are usually shallow, but in 

 some of the many variations of the species attain a considerable depth. 

 Though there is no median fold and sinus, a median anterior prolongation of 

 the valves, defined by two convergent lateral furrows, is a normal character, as 

 shown in the original figures given in the "Silurian System" (plate xix, fig. 10). 

 This siives the shell a trilobed character which is carried to an extreme devel- 

 opment in the series of shells connecting the typical form with those consti- 

 tuting the variety cylindricus. Hall. In American faunas, where this species 

 attains a great development in individuals, its numerous variations in contour 

 and general expression often possess a definite local value. The shell abound- 

 ing in the Pentamerus limestone of the Clinton group of New York is, as a 

 rule, of comparatively small size, broadly oval or obovate, rarely elongate in 

 outline ; though the trilobation of the exterior is always apparent, it is seldom 

 conspicuously defined. Rarely the shell is narrowed across the umbones, and 

 subtriangular in outline. (See Plate LXVII, fig. 2.) In New York this species 

 is not known outside of the Clinton fauna, but passing westward, it abounds in 

 the dolomites which bear a Niagara fauna in the states of Ohio, Indiana, 

 Illinois, Wisconsin and Iowa. 



At Yellow Springs, Ohio, the prevalent form is a large, elongate, usually 

 strongly trilobed shell, with narrow beaks and long, oblique cardinal slopes. 



* Siliiiian Brachiopoda, p. 153. 



