DTVERTEBltATE PALAEONTOLOGY. 22 1 



genera, both among extincl and recent mollusks. Sanguinolites anguliferus 

 of .McCoy, from the Silurian (apparently an Orthonotd), is an example of the 



latter ; while the same style of ornamentation is also seen on a species of 

 the recent genus Unio (V. bdliplicatus, Meek), from near the junction of 

 the Cretaceous and Eocene in Wyoming. In these and other similar instan- 

 ces, however, the shells are readily distinguished from Goniomya by other 

 obvious characters. 



The genus Goniomya seems to have made its first appearance during the 

 deposition of the Liassic rocks, and attains its greatest development in the 

 later Jurassic deposits. It also occurs less abundantly in the Cretaceous, 

 but appears to have died out at the close of that period ; no examples of it 

 having yet been found in the Tertiary, or among existing mollusks 



«. <> it i o in j a A 111 cricana, M & H 



Plato 30, figs. 12, a, b. 



Goniomya Americana, Meek and Haydeu (185C), Proceed. Acad. Nat. Sci. Philad., VIII, 81. 

 I'lwladomya Americana, Gabb (1861), Synop. Moll. Cret. Formation, 164. 



Our specimens of this interesting shell are too imperfect to give a very 

 clear idea of its general form; but the curve of its lines of growth indicate a 

 transversely oblong or narrow-oval outline, with a narrowly-round anterior, 

 and a subtruncated posterior margin. Laterally, the valves appear to be rather 

 compressed. The beaks are small, depressed, and placed in advance of the 

 middle. The costae are moderately distinct, though -not very prominent, and 

 at the deflection along the flanks form angles of from 20° to 30°. Those 

 near the two extremities, although converging and passing down parallel to 

 the others, do not meet to form angles along the flanks before they intersect 

 the base. The surface shows obscure concentric markings of growth, which 

 are crossed by the usual regular, equidistant, radiating rows of extremely 

 minute granules, which appear, as seen under a magnifier, like minute drops 

 of amber. The substance of the shell is very thin. 



In its surface-markings, this species resembles G. Duboisi of Professor 

 Agassiz (Etud. Crit, sur les Moll. Foss., t. 1, figs. 2-12), but its converging 

 costae continue to meet at the same acute angle as tar as they can be traced 

 toward the points of the beaks ; while on the beaks of G. Duboisi, and most 

 of the other allied species of which we have seen figures, these angles are 

 truncated, the converging costae coming down from before and behind the 



