white.] PALEONTOLOGY CRETACEOUS FOSSILS. 281 



" I have not yet seen the hinge of this shell, or its left* [right] valve, 

 and therefore 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, sharply 

 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 Bronn. It differs re- 

 markably from typical species of Avicula in its erect form, its umbonal 

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

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

 strongly incurved beak of its right [left] valve, it differs very strongly 

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

 Stoliczska thinks the characters of the genus Pseudomonotis should be 

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

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

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

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

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

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

 kind and restore both to Avicula, which I am certainly not inclined to 

 do, though I regard Oxytoma as a subgenus under Avicula. 



"I use the name Avicula here, as elsewhere, subject to the change 

 that it is probable the rules of nomenclature will demand in the restora- 

 tion of the older name Pteria, which would require the name of our 

 species to be written Pteria gastrodes, if it falls into that group." 



Subgenus Pseudoptera Meek. 



Pteria (Pseudoptera) propleura Meek. 



Plate x, figs. 2 a and &, and c. 



Avicula (Pseudoptera) propleura MEEk, 1873, An. Rep. U. S. Geol. Surv. Terr, for 1872, 



p. 489. 

 Avicula (Pseudoptera) rhytopliora MEEk, 1873, An. Rep. U. S. Geol. Surv. Terr, for 1872, 



p. 490. 



After careful examination, not only of all Mr. Meek's type-specimens, 

 which were all found associated in the same stratum, but also others 

 which I have collected at the original locality at Coalville, Utah, I am 

 satisfied that both this species and the form which he described in the 

 same publication under the name of Avicula {Pseudoptera) rhytopliora are 

 specifically identical. I therefore refer both forms to Pteria {Pseudoptera) 

 propleura, which name comes first in order in the volume above cited; 

 but I herewith give his description of both forms, together with his 

 remarks upon the same, and also figures of Mr. Meek's types of both 

 forms on plate x : 



" Shell, as determined from a left valve, obliquely ovate-subtrigonal, 

 moderately convex along the oblique umbonal slope in front of the middle, 

 and compressed-cuneate behind ; posterior margin with its general out- 

 line nearly vertical and slightly straightened along the middle, thence 

 extending obliquely upward and a little forward, with a very faint sin- 

 uosity above, to the hinge, which it meets at an obtuse angle, whfle it 

 rounds rather abruptly into the more or less rounded base below ; ante- 

 rior margin ranging obliquely backward and downward nearly parallel 



*DoTibtless an inadvertent error. Hia example ia a left valve, ■which, he describes 

 aa snch. 



