216 ACANTHOPLEURA. 



because (1) Ch. piceAis is not mentioned in Guilding's essay, and 

 therefore cannot be the type of his genus. We are not concerned 

 with the shadows of species dimly visible upon Guilding's pages, but 

 with the species actually enumerated by him, with name and refer- 

 ence. (2). Francma being founded upon the type of Guilding's 

 Acanthopleura, falls as a synonym. 



The eyes, in this genus, are generally obliterated by erosion 

 except at the edges of the valves, but they are present in young 

 individuals, as in Tonicia. They are smaller than in Tonicia, and 

 more numerous. In Mesotomura they are excessively numerous, 

 minute and oval. In Acanthopleura spiniger they are larger, cir- 

 cular, and scattered around the bases of the tubercles on the lateral 

 areas. In Maugeria granulata they are more numerous and more 

 scattered. 



The interior layer of the valves is very solid and dense, as in 

 Eudoxochiton and Tonicia, even the jugal tract inside and the slit- 

 rays lacking punctures. The tegmentum is deeply grooved over 

 the teeth and sutural-plates. 



Although but few well- differentiated species of Acanthopleura are 

 known, the range of individual and local variation is great, and con- 

 sequently an extensive literature has arisen, and the synonymy of 

 the species is exceptionally involved. It is not improbable that 

 some other valid species exist, but if so, they have not yet been 

 recognizably defined. 



The group Lucia Gld. (Lucilina Dall) has been considered a sub- 

 genus of Acanthopleura, but it proves to be a synonym of Tonicia. 



The genus Acanthopleura differs from Tonicia mainly in the duller 

 and sculptured valves, having the tegmentum more broadly inflexed 

 over the posterior margins ; in the more numerous, smaller eyes, the 

 fleshy girdle clothed with large or small calcareous spines, etc. It 

 is not especially allied to any other genus, but some species of 

 Liolophura, a genus near Onithochiton, are externally very similar 

 to species of the subgenus Maugeria. They may be distinguished at 

 once by the smooth toothless ridge in place of the insertion-plate in 

 the tail valve. 



All of the species can be readily identified by the following key, 

 whether the girdle-covering be preserved or not. It is necessary to 

 remove the posterior valve of a specimen, which may be done by 

 soaking it for some hours in warm water. The arrangement here 

 given is believed to be perfectly natural. 



