50 MOLLUSC A, 



their foot, which, instead of forming a horizontal disk, is compressed 

 into a vertical muscular lamina, which they use as a fin, and on the 

 edge of which, in several species, is a dilatation forming a hollow 

 cone, that represents the disk of the other orders. Their branchiae, 

 composed of plumiform lobes, are situated on the hind part of the 

 back, directed forwards, and immediately in their rear are the heart 

 and a small liver, with part of the viscera and the internal organs of 

 generation. Their body, a gelatinous and transparent substance 

 lined with a muscular layer, is elongated and usually terminated by a 

 compressed tail. There is a muscular mass belonging to the mouth, 

 and a tongue furnished with little hooks ; the oesophagus is very long; 

 their stomach thin ; two prominent tubes on the right side of the 

 visceral bundle afford a passage to the faeces, semen or ova. They 

 usually swim on their back with the foot upwards*. They have the 

 faculty of distending their body by filling it with water, in a way not 

 well understood. Forskahl comprised them all in his genus. 



Pterotrac HE.\, Forslc. 

 But we have been compelled to subdivide them. 



Carixaria, Lam.f 



Have the niicleus formed of the heart, liver, and organs of generation, 

 covered by a slender, symmetrical and conical shell, the point of 

 which is bent backwards and frequently relieved by a crest, under 

 whose anterior edge float the feathers of the branchiae ; two tenta- 

 cula on the head, and the eyes behind their base. 



One species, Carinaria cymhium. Lam. ; Peron, Ann. du 



Mus., XV,iii, 15 ; Poli, HI, xliv ; Ann. des Sc. Nat., tome XVI, 



pi. 1 , inhabits the Mediterranean. 



Another, the Carinaria fragi/is\ Bory Saint- Vincent. Voy. 



aux Isles d'Afr., I, vi, 4 J, is found in the Indian Ocean. 



* This mode of natation induced P(jion to think that the natatory lamina was on 

 the back, and the heart and branchiae under the belly, and has given rise to many 

 errors as respects the place of these animals. A simple inspection of their ner- 

 vous system led me to suppose, in my Memoirs on the Mollusca, that they 

 were analogmis to the Gasteropoda. A more exact anatomical investigation, made 

 sines then, with that given by M. Poli in his vol. Ill, fully confirms my supposition. 

 ITie fact is, that there is but little difference between the Heteropoda and the 

 Jectihranchiala, notwitbstanduig which, M. Laurillard believes their sexes to be 

 sep'cjated. 



f Forskahl comprised all these animals in his genus Pterotrachea, for which 

 »ame Brugi^re substituted that of Firola. Ptron Laving divded the genus, 

 appropriated the name of Carinaria to those with a shell, and that of Firola to the 

 others. Rondelet gives the Carinaria, but Tvithout its sheU. — " De Insect. Zoopb. 

 cap. XX." 



■* Add, Carinaria dep/-fS5a, Rang. Ann. des Sc. Nat., Feb. 1329, p. 136. 



