GASTEROPODA SCUTIBRANCHIATA. 257 



shell which follows its contour, and which corresponds to a similar cleft in 

 that part of the mantle which covers the branchial cavity. Along the whole 

 side of this cleft is a branchial comb, composed of numerous loose and tabular- 

 like lamellae. 



ORDER VIII. 



SCUTIBRANCHIATA. 



THE Scutibranchiata comprise a certain number of Gasteropoda, similar to 

 the Pectinibranchiata, in the form and position of the branchiae, as well as in 

 the general form of the body. Their shells are very open, without an oper- 

 culum, and most of them without the slightest turbination, so that they cover 

 these animals, and particularly their branchiae, in the manner of a shield. 

 The 



HALYOTIS, Linnceus, 



Is the only genus of this order in which the shell is turbinated ; it is distin- 

 guished from that kind of shell by the excessive 

 amplitude of the aperture, and the flatness and 

 smallness of the spire, which is seen from within. 

 This form has caused it to be compared to the 

 ear of a quadruped. In the true Halyotes, the 

 shell is perforated along the side of the columella by a series of holes ; when 

 the last hole is not terminated, it gives to that part the look of an emargination. 

 The animal is one of the most highly ornamented of all the Gasteropoda. A 

 double membrane, cut into leaves and furnished with a double range of fila- 

 ments, extends, at least in the most common species, round the foot and on to 

 the mouth; outside its long tentacula, are two cylindrical pedicles which 

 support the eyes. The mantle is deeply cleft on the right side, and the water, 

 which passes through the shell, penetrates through it into the branchial 

 cavity ; along its edges we observe three or four filaments which the animal 

 can protrude through these holes. The mouth is a short proboscis. 



In the following genera, which are separated from the Patellie, the shell is 

 perfectly symmetrical, as is also the position of the heart and branchiae. 

 In the 



FISSURELLA, Lamouroux, 



We perceive a broad fleshy disk under the abdomen, as in the Patellae, a 

 conical shell placed on the middle of the back, but not always completely 

 covering it, and perforated at its summit by a small orifice, which affords a 

 passage to the water required for respiration ; this orifice penetrates into the 

 cavity of the branchiae, situated on the fore part of the back ; a cavity other- 



s 



