180 PROCEEDINGS OF THE GEOLOGICAL sociETy. [Dec. 1, 
the apex, and then bent back, so as on the whole to be concave ; m 
such species the ventral valve is convex, beg flat near the apex, 
and then curving down regularly. But there are many species where 
both valves are nearly flat, and a few, as S. crenistria, where both 
valves are convex. 
In the large majority of Leptene the dorsal valve is convex, but 
the reverse is the case in L. evglypha and a few other species. 
We find the same irregularities in Orthis; the majority of the 
species have the dorsal valve strongly convex, and the ventral valve 
less convex or flat; but there are many species with both valves 
equally convex; O. resupinata and a few others have the dorsal 
valve concave and the ventral convex, and O. carinata, described 
above, p. 173, has the ventral valve deeply concave. Thus, though 
in each genus a certain form prevails, we cannot take these forms 
as generic characters, as there are a few aberrant species in each. 
(14) TeREBRATULA ASPERA.—This species appears to be common 
in the Hamilton and Chemung groups, and is frequently found with 
the ribs produced into spmes, one of which rises at each intersection 
of the ribs by the lamime of the shell, which occur at intervals of a 
tenth or a twelfth of an inch: the spines themselves are nearly a 
quarter of an inch long. This tendency to produce lamine project- 
ing beyond the shell appears the strongest when the animal lived in 
very still water, as we find it best exemplified in specimens from fine 
clay. Thus 7. reticularis is often found in the Wenlock shale 
covered with projecting laminz, but these never run into spines. 
The variations in appearance arising from the different degree of 
development or preservation of the scales has led to a sad multipli- 
cation of supposed species; thus the young shell of 7. retzcularis 
with the lamine slightly projecting formed the 4trypa aspera of the 
Silurian system; the preservation of the scales on the true T. aspera 
supplied the 4. sguamosa of Sowerby ; and the spose development 
of them has led to two more names in America, the 4. spinosa and 
A. dumosa ot J. Hall. : 
Another American species of the same group, 4. hystrix, Hall, 
has also the ribs produced into long spines; it is distinguished from 
T. aspera by having only about eight or ten large rounded ribs with 
larger and stronger spines; but it may perhaps prove to be merely 
a variety of that species. 
(15) TEREBRATULA UNGUIFORMIS, Conrad sp.—Mr. Conrad 
and Mr. James Hall have shown the true relations of this species by 
placing it in the genus Atrypa, which with those authors contains all 
the Terebratule found in the paleeozoic formations. Mr. Vanuxem 
has attached an undue importance to it by using it as the type of a 
new genus Hipparionyz. But as no description nor figure of the 
exterior of the shell has been published, and we have only been 
shown the cast of the interior of one valve, there has been no oppor- 
tunity given to European students of understanding its characters. 
As far as I can judge from Mr. Lyell’s imperfect specimens, the 
T. unguiformis is closely related to the T. reticularis, from which 
it differs in its greater size, coarser striation, and in the flatness of 
