EUOMPHALUS. 429 
Middleham, Clayton) fine casts of Pleurotomaria, which 
appear to be distinct from those of the upper cretaceous 
strata. 
Evompuatus.* Lign. 136, figs. 1, 2.—The shells of this 
extinct genus are deeply umbilicated, discoidal, spiral 
univalves, having the innermost whorls of the shell divided 
by imperforated partitions. The internal structure of these 
shells will serve to prepare the student for those more com- 
plicated forms of the testaceous apparatus presented by the 
Cephalopoda, which will form the subject of the next 
chapter. There are several recent univalves the animals of 
which retreat in the progress of growth from the apex of 
the spire, and the vacated portion is shut off by a shelly 
plate. In some genera a series of concave septa are thus 
formed; but in others (as Magilus) the deserted cavity 1s 
filled by a compact accretion of calcareous matter, and a 
solid elongated shell is produced. The Hwomphalus, of 
which there are many species in the Silurian, Devonian, and 
Carboniferous strata, belongs to the former group. As the 
animal increased in size, it deserted the smaller and inner- 
most portion of the spire, and a nacreous partition was 
secreted by the posterior part of the mantle, the interspace 
remaining hollow ; as this process was repeated at different 
periods, several cells were successively formed. This cham- 
bered structure is shown in the specimen Lign. 136, fig. 2, 
in which the cells are filled with spar, but the outer cavity 
is occupied by limestone like that in which the shell was 
imbedded ; a proof that no communication existed between 
the chamber occupied by the body of the animal, and the 
space from which it had withdrawn. The calcareous spar, 
as in the vegetable remains previously described (p. 71), has 
percolated the substance of the fossil, and crystallized in 
* So named by Mr. Sowerby, in allusion to the deeply umbilicated 
character of the disk. 
