GEOLOGICAL SURVEY OF THE TERRITORIES 489 



AVICULA (PSEUDOPTEEA) PEOPLEURA, Meek. 



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

 moderately convex alonjj^ the oblique nmbonal slope in front of the mid- 

 dle, and compressed cuueate behind ; posterior margin with its general 

 outline nearly vertical and slightly straightened along the middle, thence 

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

 osity above, to the hinge, which it meets at an obtuse angle, while it 

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

 terior margin ranging obliquely backward and downward nearly parallel 

 to the umbonal slope, faintly retreating near the middle, and from thiiS 

 upward to its connection with the anterior end of the hinge, projecting 

 slightly in the form of a small, short, tlatteued auricle, that is less than 

 rectangular at its extremity above, and uudetined by any marginal sinus 

 below ; hinge-line of moderate length, but not extending quite as far 

 back as the margin of the valve below it; posterior dorsal region flat- 

 tened, though not forming a proper alation ; beak rather pointed, 

 scarcely rising above the hinge, rather oblique and placed very near the 

 anterior end of the hinge, but not quite terminal. Surface ornamented 

 by moderately distinct lines of growth which, on the anterior part of 

 the valve, are crossed by seven or more slender raised radiating lines, 

 and one stronger rib that extends along tbe umbonal slope so as to give 

 it a slightly angular appearance, while very faint traces of fine radiat- 

 ing- stria3 are sometimes seen on other parts of the valve. Eight valve 

 and hinge, and interior of both valves, unknown. 



Height, measuring at right angles to the hinge, 0.00 inch ; length of 

 hinge, about 0.75 inch ; greatest anteroposterior diameter i)arallel to 

 hinge, about 0.85 inch ; length, measuring from the beak obliquely to 

 the most prominent part of the posterior basal margin, 1.20 inches; 

 convexity, about 0.23 inch. 



This species ai>pears to belong to a group of American and European 

 Cretaceous aviculoid shells that seem to me to be sufticieutly distinct 

 from the topical forms of Avicula {Pteria} and McJcagrina to stand to- 

 gether, at least as a separate subgenus. They differ from the typical 

 forms of Avicula in having no extended alations or defined byssal sinus 

 in either valve, as well as in presenting a peculiar, more or less obliquely 

 rhombic, or subtrapeziform outline. The hinge and interior of these 

 shells are nnknow^n to me, but the former seems not to be provided with 

 a gaping cardinal area, the cardinal edges being thinner and compressed. 

 Avicula anomala, of Sowerby, (183(5,) as illustrated by d'Orbiguy, in 

 Palonr. Francaise, Ter. Cret., Tome iii, PI. 392, may be regarded as the 

 tyi)e of this section, for which I would pro])ose the name Pseiuloptera. 

 It includes in addition to Avicula [Pseudoptera) anomala, Sowerby, Avi- 

 cula {Pseudoptcm) raricosta^ Eeuss, and Avicula [Pseudoptera) fibrosa, 

 Meek and Hayden. 



The two species here described are only referred to this group pro- 

 visionally, cis their right-valves are not yet certainly known. There are 

 some reasons, however, mentioned iarther on, for suspecting that this 

 valve may have a deep byssal sinus in one, if not both, of these species. 

 If this should be found to be the case, they cannot be properly referred 

 to the above-mentioned group, but would fall into a group for which 

 Stoliczka has proposed tlu^. name Electroma, typified by the recent spe- 

 cies Avicula smaragdina, Reeve, and thus have to take the name Avicula 

 [Electroma] pf'opleura, and A. [Elcctroma) rJn/tophora. Should Scopoli's 

 name Pteria, however, replace Avicula, as I believe the rules of nomen- 

 clature will require, and the section to which these shells belong, prop- 



