INVERTEBRATES. 333 



shell; anterior ear small, deeply defined; surface smooth or 

 radiately ridged; one large, faintly marked, muscular impres- 

 sion a little behind the middle; one short, narrow tooth slightly 

 diverging from the hinge on the posterior side of the beaks ; 

 ligament confined to a narrow, simple facet on the hinge 

 margin." 



In first publishing the above description in 1851, Prof. McCoy neither fig- 

 ured, described, nor cited any known shell as a type or example of his proposed 

 genus; and it was not until he republished it in 1855, and described under it 

 his S. laevigata and S- pulchella. (previously placed by him in the genus Melea- 

 grina), that Palaeontologists had the means of knowing definitely what group 

 he had proposed the name for. Hence, we can scarcely regard the genus as 

 having been established, until the date of the latter publication. 



The two species just mentioned, were alone placed by Prof. McCoy in this 

 genus, and as the last mentioned one, his S- puhhella, seems to be only known 

 from a mere fragment, giving no very satisfactory idea of either its generic or 

 specific characters, his first species, S. laevigata, must of course be regarded as 

 the type of the genus. This is a thin, compressed, smooth, Pectenoid shell, 

 with rather short, nearly rectangular ears, a short hinge line, and a very promi- 

 nently rounded, antero-ventral outline. The latter character, together with 

 the short hinge line, and position of the beaks a little behind the middle of the 

 hinge line, give the valves a rather curious backward obliquity, regarded by 

 Prof. McCoy as a generic character. His figure shows a moderate sinuosity of 

 the anterior margin, under the anterior ear, but as it was drawn from a left 

 valve, and he describes the anterior ear as being "deeply defined," we think 

 the genus may probably be found to include a somewhat wider range of forms 

 than its author seems to have intended. Hence we would place here several 

 species that have been referred by Prof. McCoy, and others, to the genus Pecten, 

 and have not, at least so strongly marked, the peculiar backward obliquity of 

 the typical species of Streblopteria The shells we allude to are smooth, little 

 Carboniferous and Permian species, and seem to have generally, if not always, 

 a deep, sharply defined byssal sinus in the anterior margin of the right valve. 

 It is possible these species may not possess the hinge characters of the type of 

 this genus, but as they certainly do not belong to the genus Pecten, as prop- 

 erly restricted to such forms as P. maximus, and appear to agree more nearly 

 in general physiognomy with Streblopteria, than with Aviculopecten, or any of 

 the other established genera, it seems proper to so dispose of them until their 

 affinities can be more clearly determined from specimens showing the hinge and 

 interior. In addition to the following described species, we would thus refer 

 to this genus such forms as Pecten dissimilis and P. consimilis, McCoy, and P. 



