22 CLASS III. GASTEROPODA. ORDER III. CERVICOBRANCHIATA. 
Patella ambigua, Chemnitz. 
Scuta antipodes, De Montford. 
Parmophorus elongatus, De Blainville. 
Emarginula elongata, Sowerby. 
Pl. CXXXIX. Fig. 4. 
PARMOPHORUS GRANULATUS, De Blainville, Bulletin des Sciences, 1817. 
Lamarck, Anim. sans vert., new edit., vol. vii. p. 579. 
Pl. CXXXIX. Fig. 5 and 6. 
ParMoPHORUS INTERMEDIUS, Nobis, Proceedings Zool. Soc., 1842. 
EMARGINULA, Lamarck. 
Testa parva, vel conica, vel depresso-conica, extus plerumque radiata, 
radiis a vertice ad marginem decurrentibus ; margine sepissimé 
crenulato, anticé fissuraé aut sinu emarginato; fissura interdum 
altiori, propé ad verticem perforata; vertice posticé subrecurvo. 
Another of the Linnean Patella, Patella fissura, was selected by La- 
marck as a type for the formation of this genus ; the slit or fissure in the 
anterior margin of the shell being considered by him as entitled to a 
generic distinction. It may be noted from our observations on this 
family, that the Fisswracea are for the most part divided into genera 
according to the change in the position of the branchial cavity. In the 
Siphonarie, for example, the communication of the water with the bran- 
chie is from the right side, and their shells exhibit a corresponding late- 
ral siphonal impression; in the Fissurelle it is from the top, and their 
shells are therefore perforated at the vertex ; in the Parmophorv it is from 
the anterior side above the head, and their shells accordingly exhibit an 
anterior siphonal impression. Now in the Emarginule the position of 
