GEOLOGICAL SUKVEY OF THE TEEEITOEIES. 491 



belong to this species.* It was nearly flat and smooth, excepting fine 

 lines of growth, aud, if I mistake not, had a tolerably deep, well-defiued, 

 byssal notch. If it really belonged to this shell, the species can hardly 

 go properly, as already stated, into the group Pseudoptera, the type of 

 which has no traces of a byssal sinus in either valve. 

 Locality and position, same as last. 



AVICULA OXYTOMA (?) GASTRODES, Meek. 



Shell (as determined from a left valve) attaining a moderately large 

 size, subtrigoual in general outline, rather distinctly convex, and having 

 a very slight backward obliquity; basal outline very profoundly rounded, 

 the deepest or most prominent part being in advance of the middle; pos- 

 terior margin moderately sinuous below the wing, from the extremity of 

 which it ranges obliquely forward and downward, rounding regularly 

 into the base below ; anterior margin strongly and subangularly sinuous 

 under the wing, thence descending with a slight forward obliquity and 

 rounding rather abruptly into the base ; hinge margin longer than the 

 height of the valve, the antero-posterior diameter of which (at any point 

 below) it also decidedly exceeds, ranging nearly at right angles to the 

 vertical axis of the shell ; beak distinctly convex, rising above the hinge 

 margin, strongly incurved, without obliquity, and situated less than one- 

 third the length of the hinge margin from the extremity of the anterior 

 wing, which is subtrigoual in form, somewhat convex, a little rounded 

 at the extremity, and very strongly separated from the abrupt shell of 

 the umbo by a deep rounded concavity extending from the beak 

 obliquely to the marginal sinus below; posterior wing longer and more 

 compressed, narrower, and more angular than the other ; both wings, 

 particularly the posterior one, projecting decidedly beyond the margin 

 of the valve below. Surface only showing more or less distinct lines of 

 growth. (Eight valve unknown.) 



Height of left valve, 1.50 inches ; length of same below the wings, 

 about 1.30 inches; length of hiuge line, 1.90 inches; convexity, (of left 

 valve alone,) 0.40 inch. 



I have not yet seen the hinge of this shell, or its left valve, and there- 

 fore have some doubts in regard to which of the sections of the old 

 genus Avicula it would most properly fall into. If the right valve is 

 (as I am inclined to think the case) nearly flat, with a deep, sharj)ly-cut, 

 byssal sinus, and its beak not distinct from the hinge margin, it will 

 probably fall into a little group for which I some time back proposed the 

 name Oxytoma, typified by Avicula Munsteri, Broun. It diflers remark- 

 ably from typical species of Avicula in its erect form, its umbonal axis 

 being inclined a little backward, instead of strongly forward. From 

 Fseudomonotis, with which it agrees in its erect form and the elevated, 

 strongly incurved beak of its right valve, it diflers very strongly in 

 having decided, well-developed ears, both in front and behind. Dr. Sto- 

 liczka thinks the characters of the genus Pseudomonotis should be ex- 

 tended so as to include Oxytoma. Should this view prevail, the name 

 of our species would probably become Pseudomonotis {Oxytoma) gostrodes. 

 It seems to me, however, that Oxytoma stands more nearly related to 

 Avicida proper than to Pseudomonotis, as typified by the Permian species 

 P. spelimcaria, so that if we unite Oxytoma to Pseudomonotis, I cannot 

 see why we might not, on the same principle, take another step of the 



* The specimen was broken to fragments in trying to detach it from the mass of 

 rock. 



