150 CLASS I. TROPIOPODA. ORDER II. UNIMUSCULOSA. 



that of Spondylus, for example : we shall, in our observations on that 

 genus (vide p. 163), be provided with the means of showing it more 



distinctly. 



Examples. 



PI. CVIII. Fig. 1 to 4. 



Vulsella lingulata, Lamarck, Anim. sans vert., new edit., vol. vii. 



p. 267. Enc. Me'th., pi. 178. f. 4. De Blainville, Manuel de Ma- 



lacologie, pi. 62. f. 5. 

 Mya vulsella, Linnaeus. 

 Ostrea vulsella, Bruguiere. 



AVICULA, Klein. 



Testa irregularis, fibro-laminaris, inaequivalvis ; basi transversa, recta, 

 lateribus nonnunquam brevibus, latere antico ssepe admodum prae- 

 longo. Lacuna pro bysso ad basem valvae sinistra?. Cardo eden- 

 tulus ; sed tuberculo dentiformi in utraque valva infra umbones 

 plerumque instructus. Area ligamentifera marginalis, linearis, an- 

 gusta, in medio dilatata. Musculus compositus, impressione magna 

 subcentrali, caeteris minutis in seriem interruptam deorsum decur- 

 rentibus. 



In accordance with the opinion of De Ferussac, Sowerby, De Blainville, 

 Deshayes and others, we include under the present genus both the Aviculae 

 and MeleagrincE of Lamarck. The types of these divisions, when separately 

 considered, are forcible and distinct ; but their generic value is lost not 

 only by the discovery of the intermediate varieties of their shells, but 

 also, according to Poli, by a perfect analogy of organization in their animal 

 inhabitants. The peculiar wing-shaped shell in the typical species of the 

 first division attracted the attention of naturalists long before the time of 

 Linnaeus ; they were described by these authors under the title of Cochlea 

 aliform.es ; and even that of Avicula may be traced as far back as Klein. 

 Linnaeus, however, included them with the Mytili ; he was an enemy to 



