BRACHIOPODA. 257 



The muscular area is less cleai'ly delimited than that of the opposite valve, 

 and its component scars are not often distinctly defined. It is, however, broader 

 and longer than on the pedicle-valve, and is divided transversely into anterior 

 and posterior adductor scars, the former being the larger, and their surface cov- 

 ered by branching lines radiating from a median longitudinal ridge. From the 

 narrow and somewhat elevated posterior extremities of this area extends a 

 broad median sinus, on either side of which arises a stout vascular trunk 

 bending backward and over the post-lateral slopes. Secondary branches are 

 given oflF from both its margins. These vascular markings of the genital region 

 are rarely well defined. 



Surface of the shell distinctly plicated, with sparse concentric wrinkles near 

 the anterior margin. The radial lines may be fine or coarse, but no species is 

 known in which the surface is entirely smooth.* 



Shell-substance punctated beneath the epidermal layer. 



Type, Terebratula ovoides, Eaton. Oriskany sandstone. 



Observations. The discussion of the genus Amphigenia has given occasion 

 to notice its close resemblance in form and various structural details to the 

 typical Renssel^ria, or properly, to the Rensselgerias most closely associated 

 with that genus in time. The existence of a spondylium in the one, and of a 

 terebratuloid brachidium in the other, are the distinctive differences in the 

 two groups. 



A considerable number of species has been referred to the genus Rens- 

 seLjEria, but it seems necessary to remove some of them to other genera 

 on account of significant structural differences; and among those which 

 remain to Renssel^ria in this somewhat restricted sense, there are still some 

 variations of a chronological or developmental value. In the type species, R. 

 ovoides, we are dealing with a shell in which size, form and structural features 

 have been carried beyond the normal mature or acmic stage of phyletic devel- 

 opment. This becomes evident from a comparison with the earlier species 



*The species R. IcBvis, Hall, from tlie Sliiily limestone of the Lower Heldeibei-g gioiip, was described 

 as having- a smooth exterior, and should this pioVe to be the fact the shell will probably prove not to belong 

 to this genus. Its interior is still unknown. 



