BRACHIOPODA. 387 



Stropboiuena incurvata."] 



cardinal area along each side of the chilidium, with the thin, erect, crural plates 

 forming their inner walls. The space between the crural plates is slightly thickened 

 and occupied by a short, strong, bilobed cardinal process. Its upper surface is trans- 

 versely striated and has a shallow median depression along each lobe. The rostral 

 thickening is continued forward but a short distance and converges to a low median 

 ridge which separates the two large, shallow scars of the adductor muscles. In front 

 of the latter are sometimes seen two, linear, slightly diverging ridges, probably the 

 markings of the main trunks of the vascular system. Genital markings on each 

 side of the muscular scars, consisting of series of tubercles of various sizes radially 

 arranged. Surface near the periphery more or less distinctly marked by numerous, 

 short, irregular, radiating striae, much the strongest in the medial region. 



The variations of this species are mainly those of convexity, thickness of shell 

 and alternation of striae. In the "Lower Blue beds" of Wisconsin, where this species 

 is abundant, the alternation of striae is a very persistent character and the valves 

 are usually flatter than specimens from other regions. 



In the Trenton shales of Minnesota S. incurvata is also a common species, often 

 preserving the delicate markings of the interior. The variation in convexity here 

 attains its maximum, and while the striae likewise alternate in size, this feature is 

 never so conspicuous as in Wisconsin. 



This species is usually known as S. filitexta Hall. It is thought that to this form 

 Shepard applied the name S. incurvata nine years prior to that given by Prof. Hall. 

 One of the writers, through Mr. John M. Clarke; has endeavored to find the types at 

 Amherst College, but Prof. Emerson states that no specimens of it with Shepard's 

 label attached exist at present in that collection. While the original description and 

 illustrations are not very satisfactory, yet sufficient is shown, combined with the 

 locality, to warrant the conclusion that the above specimens were of this widely 

 distributed species. Strophomena convexa Owen, proposed three years earlier than 

 Hall's name, is undoubtedly a specimen of the same species, which was derived from 

 the " Blue and Grey limestone of Wisconsin and Iowa." Probably Owen subsequently 

 regarded it as identical with some other form, for in a subsequent report (1852) no 

 mention is made of it. 



This species is commonly stated to occur in the Hudson River group, but a com- 

 parison of specimens from that horizon with those from the Trenton will show the 

 interior of the dorsal valve of the former to be entirely different in its prominent 

 vascular ridges, while the space beneath the cardinal area on each side of the teeth 

 in the ventral valve is filled up by a deposit of shell matter. Since Mr. U. P. James 

 has applied the name Strophomena neglecta to one variation of the species identified 

 by Meek in 1873 as S. filitexta, it is advisable to refer to the specimens from the 



