282 REPORT UNITED STATES GEOLOGICAL SURVEY. 



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

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

 slightly in the foim of a small, short, flattened auricle that is less than 

 rectangular at its extremity above and imdeflned by any marginal sinus 

 below ; hiage-Hne of moderate length, but not extending quite as far back 

 as the margin of the valve below it ; posterior dorsal region flattened, 

 though not forming a i^roper alation ; beak rather pointed, scarcely ris- 

 ing above the hinge, rather oblique, and placed very near the anterior 

 end of the hinge, but not quite terminal. Surface ornamented by mod- 

 erately 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 the umbonal slope, so as to give it a 

 slightly angular appearance, while very faint traces of fine radiating 

 strijB are sometimes seen on other parts of the valve. Eight v<alve and 

 hinge and interior of both valves unknown. 



"Height, measuring at right angles to the hinge, 0.90 inch; length of 

 hinge, about 0.75; greatest antero-posterior diameter parallel to the 

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

 most prominent part of the postero-basal margin, 1.20 inches ; convexity, 

 about 0.23 inch. 



" This species appears to belong to a group of American and European 

 Cretaceous Aviculoid shells that seem to me to be sufilciently distinct irom 

 the tyi>ical forms of Avicula {Pterin) and Meleagrina to stand together, 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 subtrapezoid outline. The hinge and interior of these shells are un- 

 known to me, but the former seems not to be i^rovided with a gaping 

 cardinal area, the cardinal edges being thinner and compressed. Avicula 

 anomala of Sowerby (1836), as illustrated by d'Orbigny in Paleont. Fran- 

 9aise, Terr. Cret., tome iii, pi. 392, may be regarded as the type of this 

 section, for which I would propose the name Pseudoptera. It includes, 

 in addition to Avicula {Pseudoptera) anomala Sowerby, Avicula {Pseudop- 

 tera) raricosta Eeuss, and Avicula {Pseudoptera) fibrosa Meek & Hayden. 



"The two species here described are only referred to this groui) provi- 

 sionally, as their right valves are not yet certainly known. There are 

 some reasons, however, mentioned further 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 Stoliczska 

 has proposed the name Mectroma, typified by the recent species Avicula 

 smaragdina Eeeve, and thus have to take the name Avicula {Electroma) 

 propleura and A. {Electroma) rhytophora. Should Scopoli's name Pteria, 

 however, replace Avicula., as I believe the rules of nomenclature will 

 require, and the section to which these shells belong properly fall into 

 that genus, either as a subgenus or otherwise, then the name Pteria will 

 have to be substituted for Avicula in connection with these species. 



'■^Locality and position. — CoalviUe, Utah ; from white sandstone, 250 

 feet above the lower heavy bed of coal mined at that place." 



TLe following is Mr. Meek's description of and remarks upon the form 

 which he designated as Avicula {Pseudoptera) rhytophora : 



^' Shell, as determined from a left valve, but slightly oblique, rhom- 

 bic-suboblong, and nearly twice as high as wide in adult exami}les, but 

 proportionally broader and subtrigonal in young specimens ; moderately 

 convex, the greatest convexity being toward the anterior side, along the 

 umbonal slope, which appears to be angular, thence cuneate i)osteriorly, 



