BRACHIOPODA. 
257 
The muscular area is less clearly 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 off 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 Rensselaerias most closely associated 
with that genus in time. The existence of a spondylium in the one, and of a 
terebratuloid brachidiurn in the other, are the distinctive differences in the 
two groups. 
A considerable number of species has been referred to the genus Rens- 
SEL^RiA, 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. 
ovoid,es, 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 1{. IcBvis, Hall, Irom the Shaly limestone of the Lower Heldei-bei'g- gioup, was described 
as having a smooth exterior, and should this prove to be the fact the shell will probably prove not to belong 
to this genus. Its interior is still unknown. 
