134 CLASS I. TROPIOPODA. ORDER II. UNIMUSCULOSA. 



was very wide in some, whilst in others it was nearly closed, established 

 a still further subdivision. There are other peculiarities, however, in the 

 shell of Hippopus which have induced us to retain this genus : the outer 

 surface is curiously imbricated, and the interior is lined with clear white 

 enamel, transparent as alabaster ; the muscular impression, too, is not 

 spread out towards the ventral margin, as in the shell of Tridacna. 

 These, it may be said, are not very important differences, but they are 

 peculiar and unchangeable ; the size of the lunular opening cannot be 

 entirely relied on as a generic character, but in this genus it never varies, 

 being always very small, whilst in the genus Tridacna it is generally very 

 large. In fact, it is only in the shell of the great Tridacna gigas that the 

 lunular opening is known to be nearly filled up ; and this arises from the 

 excessive increase of the valves, which may probably require centuries to 

 complete their growth. 



The shell of Hippopus may be described as being regular, equivalve, 

 inequilateral, and imbricated on the outside with numerous tubercles. 

 There is but a slight opening in the lunula, the animal having a very 

 small foot ; and it is moreover not supposed to be furnished with any 

 byssus. The hinge consists of two compressed unequal teeth in each 

 valve, the anterior of which are inserted. The impression of the muscle 

 is very nearly central, and the ligament is external and marginal. 



Examples. 



PI. XCVIII. Fig. 1 . 



Hippopus maculatus, Lamarck, Anim. sans vert., new edit., vol. vii. 



p. 12. Enc. Me'th., pi. 236. f. 2, a, b. 

 Chama hippopus, Linnaeus. 

 Tridacna hippopus, De Blainville. 

 Tridacna rnaculata, Quoy. 

 Hippopus brassica, Schumacher. 



PI. XCVIII. Fig. 2. (fossil.) 

 Hippopus avicularis, Sowerby, Genera of Shells, No. 13. 



