HIPPOPUS. 51 
would almost lead to the belief of its being an attached shell. 
Fig. 173. H. Pusio. 
HIP’PAGUS. Lea. (Horse Boat.) A minute fossil resembling 
Isocardia in form, but destitute of hinge teeth. H. Isocardivides, 
fig. 128. 
HIP POCHRENES. Montf. Species of Rostetiarim, Auct. with 
the outer lip spread. Fig. 403, R. Columbaria. 
HIP’PONYX. (inmos, hippos, horse ; ovv£, onyz, nail or hoof.) Fam. 
Rudistes, Lam. Descr. Inequivalve, sub-equilateral, rather irre- 
gular, destitute of ligament and hinge teeth; lower valve at- 
tached, flat, sub-orbicular, with a muscular impression, composed 
of two lunulate portions, meeting at one extremity, and pre- 
senting the form of a horse-shoe; upper valve conical, with 
the apex inclined backwards, and the muscular impression mar- 
ginal. Cbs. The earlier naturalists having met with only the 
upper valve of these shells, placed them among the Patelliform 
univalves; tosome of which, particularly Pileopsis, they beara very 
strong resemblance. The species of Hipponyx are numerous, 
and till lately only known in a fossil state. Fig. 199, H. Cor- 
nucopia. 
HIPPOPO'DIUM. Conybeare. Fam. Cardiacea, Lam. Deser. 
Equivale, obliquely transverse, ponderous, deep, inequilateral ; 
umbones incurved; ventral margin sinuated, so as to give a bi- 
lobed appearance to the shell; hinge incrassated, with one 
rugged, oblique tooth. Obs. These fossils are found in the 
upper beds of Lias. Fig. 129, H. Ponderosum. 
HIP PURITES. Montf. Fam.Orthocerata, Lam. Rudistes,Bl. Deser. 
Tubular, rude, irregular, attached; lower valve cylindrical, 
more or less lengthened, apparently divided into sections by septa, 
(considered by some authors as merely projecting layers 
of growth,) having one or two lateral tubes within; upper valve 
round, flat, fixed on the aperture of the tubular valve, like an 
operculum. This genus is known only in a fossil state, and but 
very imperfectly. Lamarck places it among his chambered Ce- 
phalopoda, and Blainville, considering it a true bivalve, enume- 
rates it with his Rudistes. Hippurites Cornucopia, fig. 198. 
HIP’POPUS. Lam. (Inzos, hippus, horse; mus, pus, foot.) Fam. 
Tridacnea, Lam. Wescr. Equivalve, inequilateral, regular, sub- 
quadrate; lunule closed, flat, with crenulated edges; ventral 
margin deeply undulated; external surface, fluted with radiating 
ribs, which are transversely fringed with rows of tubular 
spines; hinge margin thick, with two long, compressed, poste- 
rior lateral teeth in one valve; three in the other; ligament mar- 
ginal, external. Obs. The shell thus described is rightly sepa- 
rated from Tridacna, on account of the anterior dorsal margins 
being closed; whereas in Tridacna there is a wide hiatus. Only 
one species of this genus is known, which receives its name 
