SPONDYLUS. PLAGIOSTOMA. 399 
In the cretaceous strata of North America, Dr. Morton 
has discovered a Spondylus (S. dumosus) very nearly 
related to S. spinosus; but it differs in its general form, and 
has both valves beset with strong spines. I have the frag- 
ment of a large bivalve from the Kentish Rag (Mr. Bensted’s 
Lien. 128. Sronpytus spinosus. In Chalk-flint. Lewes. 
quarry), which has the peculiar structure of the Water- 
clam (Spondylus varius of Mr. Broderip); namely, hollow 
interspaces formed by shelly layers or partitions, which 
were secreted by the posterior part of the mantle, or in- 
vesting integument of the animal, as it gradually receded 
from that part of the shell. In the recent Water-clam the 
cells are full of fluid.* 
Practostoma, Llhwyd, 1699. This genus, adopted by 
Mr. Sowerby in the Mineral Conchology, is scarcely distin- 
guishable from Lima of Bruguiere (1791). Most of the 
recent species are ornamented with small asperities, from 
which the name lima (file) is derived ; they are symmetrical 
shells attached by a byssus. 
* See Penny Cyclop. Art. Spondylide. 
