RHYNCHONELLIDiE OF 'THE TULLY LIMESTONE. 
347 
which are often obscurely defined. In old shells there are forty or 
more rather broad, rounded, moderately elevated plications, of which 
from six to eight are depressed in the sinus, and a corresponding num¬ 
ber elevated on the mesial fold. These, as well as some of the lateral 
plications, are usually bifurcated above the middle, but occasionally 
below. Those near the margin, and particularly on the fold and sinus, 
are flattened and longitudinally grooved. The plications are crossed 
by fine concentric striae which are not visibly imbricating. 
The casts show a small rostral cavity, bordered by short and rather 
thin dental lamellae, with an elongate-ovate muscular impression, and a 
moderately or scarcely elevated septum in the dorsal valve. The surface 
shows a beautiful ramified system of vascular markings, the main trunks 
of which curve around the papillose ovarian spaces, and thence bifurca¬ 
ting are distributed according to the divisions of the plications, giving 
a branch to each one, and these again usually subdivide towards the 
extremities. The central and subordinate lateral groovings of the exte¬ 
rior of the plications correspond to the vascular markings and their 
subdivisions. 
The smaller specimens measure sometimes less than half an inch in 
length and breadth; while the full-grown shells measure from seven- 
eighths to nearly an inch in length, with a width sometimes exceeding 
an inch, and a depth frequently of three-fourths of an inch. 
This beautiful fossil occurs in considerable numbers in several localities of the 
Tally limestone, and in full-grown individuals is readily distinguished ; but the 
immature phases are not so easily recognized. 
This species was identified by Mr. Vanuxem with the Terebratula cuboides of 
Sowerby ; and the same name with doubt was used in the Report of the Fourth 
District. M. A. D’OrbAhvy has separated the species originally -described by 
Sowerby from that described under the same name by Philips (loc. cit.), propo¬ 
sing the name Atrypa subcuboides for the latter, citing at, the same time the 
American species as identical. 
The British species has a different aspect and has more plications on the mesial 
fold and sinus, which is proportionally more elevated in front; while in the exfo¬ 
liated shell all the plications are more angular. It becomes necessary, therefore, 
to propose a distinctive name for the American species. Although having a gene- 
