1¥2 GEOGNOSY AND GEOLOGY. 
teeth, of a V shape. Trigonia navis ( fig. 39) characterizes the upper lias, 
and T. clavellata ( fig. 40) the Oxford marl. 
Pholadomya has a thin shell, open on both sides, with opposed beaks, but 
no teeth. P. exaitata ( fig. 41) is peculiar to the Oxford elay. * 
Diceras possesses a very thick shell, running out into sub-spirally twisted 
beaks, and a hinge whose ridges exhibit some resemblance to a human ear: 
D. arietina ( fig. 42). 
Astarte has nearly circular valves shutting close together, and two 
cardinal teeth. It fills entire beds of the coral-rag, known as the Astarte 
limestone: A. elegans ( jig. 48). 
The Nucule, shells belonging to the family Arcacee, are extensively 
distributed in the Jurassic strata. They are small, regular shells, with four 
toothed hinges: Nucula hammeri (fig. 44, a and b). The genus Pinna is 
an elongated equivalve shell, whose beaks end in a point, and whose hinge 
margin is toothless. P. hartmanni (fig. 45) is found in the lower las. 
Pterocera oceani (fig. 46) is peculiar to the Portland limestone. This 
genus possesses a thick shell anda short axis. The oral aperture is narrow, 
and runs out above into a long canal. The external lip is expanded into a 
digitated wing. The Pleurotomaria have a thick, conically spiral shell, and 
a narrow quadrangular or rounded aperture, with a deep incision in the 
external margin, which distinguishes it from Trochus: P. conoidea ( fig. 
47) from the lower oolite. 
Nerinea, a gasteropod genus, peculiar, excepting in one species, to the 
Jura, has a long conical, often cylindrical, spiral shell with very thick walls, 
and having highly characteristic internal folds. Certain species occur in 
great numbers of individuals, as IV. suprajurensis (fig. 48), one turn of 
which is exposed in longitudinal section. IV. mose (fig. 49), and NV. 
godhallu (fig. 50); fig. 51 is a longitudinal section of the latter. The 
most conspicuous Ammonites are Ammonites bucklandii (fig. 52) in the 
lias ; A. catena (fig. 53), also in the lias, and A. striatulus (fig. 54) in the 
lower oolite. 
There are a few species of the Nautilus family, as Nautilus lineatus (fig. 
55), in the lower oolite. Here, as also in the muschelkalk, we find the 
beaks of cephalopods, known as Rhyncholites, or Conchorhynchs. These 
Rhyncholites exhibit a striking similarity to the mandibles of parrots or of 
Chelonie. PJ. 38, fig. 56, is a lateral view of one of these Rhyncholites. 
The fossil known as Belemnites, belonged, in all probability, to a 
cephalopod, and, like the so-called ossa sepi@, was an internal framework 
inclosed by the softer parts of the animal. They are generally conical 
bodies of various shapes, mostly of a crystalline texture, the crystals 
radiating from the longitudinal axis of the Belemnite (fig. 58). They run 
out below into a more or less acute point, and at the upper part exhibit a 
conical excavation. The ink bag of the Belemnite has been found in this 
cavity in a state of preservation sufficient to admit of the use of the contents 
in drawing the fossil itself. The internal structure of the cavity may 
sometimes be ascertained in the upper portion: in it was inserted an 
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