GEOGNOSY AND GEOLOGY. 151 
with a round aperture, through which passes the attaching ligament ; under 
this lies the triangular cardinal area, bordering beneath on the cardinal 
margin. The most common species are Terabratula ferita ( pl. 37, fig. 26), 
T. wilsonii ( figs. 27 and 28), affinis (pl. 42, fig. 59), crispata (pl. 37, fig. 29), 
and imbricata (fig. 30). In the Devonian System we have Caceola 
sandalina of conical shape, provided with an operculum beneath 
( figs. 31 and 32); also Producta depressa (pl. 42, fig. 58). 
The Gasteropoda are readily distinguished from the preceding by their 
external form. The shells of this family are twisted from left to right 
(dextral), sometimes, however, in the opposite direction (sinistral), and 
generally consist of one, rarely of two valves. In them we distinguish an 
apex and a base, in the latter of which is the orifice through which the 
animal protrudes itself. The axis around which the spiral cone is wound is 
called the columella or spindle; this is generally solid; when hollow, the 
aperture of the space included, is called the umbilicus. Some gasteropods 
possess an operculum by which the opening can be closed after the 
retreating animal. 
The family of Turbinites is characterized by a turriform, conically wound 
shell, with the mouth entire and rounded. The columella is curved, and 
ends in a small open umbilicus. The species of the genus Turbo have 
mostly beautifully ornamented shells. The most important species is Turbo 
squamiferus (pl. 37, fig. 33). Monodonta is allied to Turbo ; in this genus 
the columella ends at the aperture in a projection or tooth: Monodonta 
purpurea ( fig. 34). The genus Natica also belongs here ; its species have 
the spire depressed, the aperture ovate, with a trenchant right border, a 
callosity masking the umbilicus: N. subcostata. Pleurotomaria has a 
conical spire with oblique oval aperture; P. defrancei (fig. 35), P. llovdit 
( fig. 36). Euomphalus possesses an inconsiderable spire, sometimes none 
at all; the aperture perfect, with angular border; umbilicus smooth: E. 
rugosus (fig. 37), EH. discors (pl. 42, fig. 57), EH. serpula (pl. 37, fig. 38). 
Cirrhus is distinguished from the preceding by the conical elevation of the 
spire, C. leonhardii ( pl. 37, fig. 39); in Schizostoma the turns of the spire 
lie in a plane, S. radiata (fig. 30). Among the gasteropods, with twisted 
columella and without umbilicus, belong Buccinum: B. aculeatum (fig. 41), 
and Murchisonia, differing from the last in the oblique oblong aperture, 
ending in a short canal, and by the ridge which follows the windings: 
M. coronata (pl. 37, fig. 42). Bellerophon has a shell curved like Nautilus, the 
last winding entirely concealing all the rest; it is, however, not divided into 
chambers, neither does it possess a sipho: Bellerophon bilobatus ( fig. 43). 
Gasteropods also occur without a twisted or curved shell, as in Conulara: 
G. gervillei (fig. 44). They are tri- or quadrilateral, and narrow above. 
The half-closed mouth is placed in the base. 
The Cephalopoda stand at the head of the Mollusca in respect to their 
organization. The head, which is furnished with two well formed eyes, is 
distinct from the body. The mouth is placed in a depression of the head, 
and contains two strong jaws bearing a somewhat striking resemblance to 
the beak of a parrot. It is surrounded by a variable number of long 
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