164 CLASS I. TROPIOPODA. ORDER II. UNIMUSCULOSA. 



shell, and a longitudinal groove is formed down the centre by the advance 

 of the ligamentary cavity. This groove is always partially filled with the 

 decayed remains of the original ligament ; in some species it is exposed 

 (vide PI. CXVII. f. 3.) ; in others it is thinly covered over (vide PI. CXVIII. 

 f. 6.). The hinge consists of two solid bent teeth in each valve, strongly 

 interlocking with each other, and the cavity containing the ligament comes 

 exactly between them. The ligament is very strong and solid, and by the 

 aid of this, with the interlocking teeth, the valves are very firmly united ; 

 Sowerby asserts that there is also a slender portion of ligament running 

 along the hinge margin. The muscular impression is sublateral. 



There has always been considerable difficulty in determining the species 

 of this much admired genus ; nearly all the varieties, indeed, that were 

 known to Linnaeus and his contemporaries were referred to the Spondylitis 

 gcedaropus. Their shell becomes very variably modified, in fact, often 

 distorted, by their peculiar situations of growth, and the spines or folia- 

 tions are thrown out in great abundance to afford facilities for their 

 attachment. The valves, like those of the Pectines, are sometimes alike, 

 as in the Spondylus regius ; sometimes different, as in the Spondylus Ame- 

 ricanus ; but in these latter the upper valve is usually the richer both in 

 colour and ornamental development. 



Examples. 

 PL CXVI. Fig. 1. 



Spondylus Americanus, Lamarck, Anim. sans vert., new edit., vol. vii. 



p. 185. Enc. Meth., pi. 195. f. 1 and 2. Chemnitz, Conch., vol. vii. 



pi. 45. f. 465. 

 Spoiidylus gcedaropus (var. 6.), Gmelin. 



PI. CXVI. Fig. 2. 

 Spondylus princkps, Broderip, Proceedings Zool. Soc, 1833, p. 4. 



PI. CXVII. Fig. 3. 

 Spondylus g/edaropus, Linnaeus, Syst. Nat., p. 1136. Enc. Me'th., 

 pi. 190. II. a, b. Chemnitz, Conch., vol vii. pi. 44. f. 459. 



