HYALvEA. 163 



HIPPOPUS. Lam. ('Ittttoc, hippus ; ttovg, pons, foot.) Fam. 

 Tridacnacea, Lam. — Descr. 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 posterior lateral 

 teeth in one valve, three in the other ; ligament marginal, exter- 

 nal. — Obs. The shell thus described is rightly separated 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 from its resemblance 

 in form to a horse's foot, when held with the flat anterior dorsal 

 margin downwards. Few shells are found to concentrate so many 

 beauties as the Hippopus Maculatus, commonly called the Bear's - 

 paw- clam ; the delicate whiteness of the interior, the undulating 

 edge, the radiated fluted columns, adorned at intervals by crisped 

 fringes, and the richness of the variegated colouring, are such as 

 to secure the admiration of the most superficial observer. From 

 the Indian Archipelago. Fig. 156, H. Maculatus. 



HIPPURITES. Montf. Fam. Orthocerata, Lam. Rudistes, Bl. 

 — Descr. Tubular, rude, irregular, attached ; lower valve cylindri- 

 cal, 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 oper- 

 culum. — Obs. This genus is known only in a fossil state, and but 

 very imperfectly. Lamarck places it among his chambered Cepha- 

 lopoda, &c. De Blainville, considering it a true Bivalve, enume- 

 rates it among his Rudistes. Cretaceous group. Fig. 198, H. 

 Cornucopia. 



HORTOLUS. Montf. Spirolina, Lam. Microscopic. 



HYALiEA. Auct. (Hyalus, glass.) Fam. Pteropoda, Lam. 

 Thecosomata, Bl. — Descr. Globose, glassy, transparent, with a 

 triangular opening at the upper part where the dorsal portion 

 advances beyond the ventral ; ventral portion vaulted ; dorsal 



m 2 



