BIVALVES SPONDYLUS. 47 



SPONDYLUS Thorny Oyster or Artichoke. 



DESCRIPTION OF PLATE XI. 



DiT. I. — Fam. 1. Fig. 1. S. gaedaropiis. — Fig. 4. varietj'. 

 Dnr. II. — Fam, 1. Fig. 3. S. anacanthus. Fam. 2. Fig. 2. S. plicatus. 



Shell hard, solid, with unequal valves; one of the valves convex, the 

 other rather flat: hinge with two recurved teeth separated hj a 

 small hollow. 



THIS genus, though presenting innumerable varieties, 

 contains only ten distinct species, and those, from 

 their extreme irregularity of formation, and gieat simi- 

 larity of appearance, may often be confounded with each 

 other. 



The most striking character of the Spondylus is, that 

 its valves, which resemble those of the common oyster, 

 have their outsides covered with longitudinal rows of 

 erect spines or ramifications. The spines are usually 

 round, and terminate in a point ; the ramifications or 

 branchings, on the contrary, are flat, and have jagged 

 and patulous extremities. These peculiarities form the 

 distinctions of the two families of the first division. 

 Many species possessing these characters are also con- 

 centrically plaited on the lower valve. The second 

 division is distinguished by its species being longitudi- 

 nally striated or plaited. 



The hinge is furnished with two recurved teeth, which 

 are very strong and articulate, and separated by a small 

 but deep hollow ; the inner margin is usually crenulat- 

 ed, and highly colored with orange or purple. TJie 

 lower valve generally proti*udes much beyond the upper, 



