M O L L U S C A. 



627 



The sexes are united in the same individual in such 

 a way as not to require union with another ; or rather, 

 the male organs are unknown. The heart is traversed 

 by the rectum, and is furnished with two auricles. The 

 cloak is protected on the back by a conical or subspiral 

 chelly-shield. into which the animal is capable of with- 

 drawing itself. The foot is destitute of a lid. The 

 following genera have been established in this order. 



1. HALYOTIS. The shells of the animals of this 

 genus have frequently been described, but it is to Cu- 

 vier that we owe the most accurate details concerning 

 the structure of their inhabitants. The foot is oval 

 and large. The sides of the body all round are orna- 

 mented with one or more rows of simple or branched 

 filaments. The shell is placed on the back with the 

 spiral part behind, and the row of holes on the left side, 

 through which some of the filaments are protruded. 

 The animal is attached to the shell by a single large 

 muscle. The entry to the branchial cavity, which 

 likewise contains the termination of the rectum and 

 oviduct, is on the back. The gills are in two ridges, 

 consisting of complicated branched filaments. At the 

 entrance of the cavity, the cloak it furnished with a 

 slit, the left margin of which rests upon the pillar of 

 the shell. The edges of this slit are furnished with 

 filaments, which pass through the anterior holes of the 

 shell. The use of this singular arrangement is un- 

 known. The branchial cavity likewise contains the 

 viscous organ, in common with the Pectin ibranchia?. 



The hood is emarginate, with a long tentaculum on 

 each side, behind which, towards the side, is a cylin- 

 drical protuberance, bearing the eye at the top. The 

 mouth is in the form of a short proboscis, with two 

 corneous plates as cheeks, and a long narrow tongue 

 extending backward*, and covered with spines. The 

 pharynx is dilatable, with internal folds. The sali- 

 vary glands are very small. The gullet is very short. 

 The stomach is divided into two portions. The first 

 is striated longitudinally with a glandular structure, 

 and receives a biliary duct. The second is separated 

 from the former by a valve, is smaller, with transverse 

 stria;, and a double ridge. It likewise receives bile 

 through two apertures. There is another valve at the 

 pylorus ; and the intestine, after making some turns, 

 is surrounded by the heart. There is an auricle on 

 each side, receiving the aerated blood from each of the 



gilt'- 



The species of this genus are by no means numerous. 

 Montfort separated those in which the marginal holes 

 were nearly obliterated, with an internal groove and 

 external ridge in the line of their direction, and formed 

 them into a genus which he terms Padolles. Lamarck 

 likewise separated those whose shells are destitute of 

 the marginal holes, under the generic appellation Sto- 

 matia. 



2. CAPULCS. This genus is represented by the Pa- 

 tella hungarica. The foot is complicated on its ante- 

 rior margin. The shell adheres to the animal by a cir- 

 cular muscle, leaving an opening in front, for the issue 

 of the head and entrance to the branchical cavity. The 

 gills form a single ridge across the roof. The mouth 

 is in the form of an extended proboscis, with a deep 

 groove above. The tentacula, which are two in num- 

 ber, have the eyes at the external base. The anus is 

 on the right side of the branchial cavity. 



We possess two species of this genus in our seas, 

 Hungarica and Antiquata, to which M. Cuvier is dis- 

 T to add the BulLa velutina of Midler. This last 



Eunirginu- 

 la. 



shell, however, we have already noticed as belonging 

 to the Pectinibranchiae. 



3. FISSURELLA. The animals of this genus have the 

 foot like the preceding, and the back is protected by a 

 conical shell perforated at the apex. The shell is united 

 to the cloak by a circular muscle open in front. The 

 eloak forms a duplicature in front for the branchial ca- 

 vity which extends to the perforated apex of the shell. 

 The gills consist of two ridges ; at the dorsal extremity 

 of which is the anus. It is probable that the excre- 

 ments are ejected at the perforation in the apex of the 

 shell, and likewise the water which enters the branchial 

 cavity in front. The head is furnished with two ten- 

 tacula bearing the eyes at the external base. 



The Patella graeca and apertura, may be quoted as 

 British examples of the genus. 



4. EMAROIN ULA. This genus differs from the former 

 in the apex of the shell not being perforated. Its place, 

 however, is supplied by a slit on the anterior margin, 

 the entrance to the branchix and anus. The foot is 

 surrounded with a row of filaments, and the eyes are 

 supported on short foot-stalks, characters in which it ap- 

 proaches the genus Halyotis. The Patella fissura of 

 conchologists is .considered as the type of the genus. 



5. CREPIDULA. The shell of this genus is conical, with Crcpidula. 

 the apex obliquely directed to the right side behind. 



From the hinder part of the cavity there is a thin plate 

 or horizontal projection, which serves as a support to 

 the abdomen. The gills form a transverse ridge on the 

 roof of the cavity. They consist of simple filaments 

 which extend considerably beyond the margin ot the 

 cavity. The eyes are placed on the head, at the base 

 of the tentacula. The Patella fornicata is the type of 

 the genus. 



6. NAVICELLA. The shell in this genus differs from Navicella. 

 the former in the apex being mesial, and the horizontal 



plate smaller. The abdominal sack likewise contains a 

 loose, irregular, angular, testaceous body, a character of 

 which there is no other example among the gasteropo- 

 da. The species appear to be fluviatile, living in the 

 rivers of warm countries. The Patella neritoidea is 

 the type of the genus. 



7. CALYPTRKA. The animal of this genus is not Calyptrea. 

 known. But the characters of the shell are well marked. 



It is conical, with a spiral lamina descending from the 

 apex in the interior, marking the commencement of a 

 pillar. The Patella chinensis of British writers may be 

 quoted as an example of the genus. 



8. CARtNARiA. This genus was instituted by Lamarck, Crinri. 

 for the reception of the shell termed Argonauta vitrea. 

 The animal has hitherto been imperfectly examined. 

 The foot appears to be compressed, and formed for 

 swimming. The head is covered with a group of tu- 

 bercles. The mouth is furnished with a proboscis. 

 Near the middle of the body, the shell is attached. The 

 surface of the body above is closely covered with 

 small tubercles. It is probable that the species here al- 

 luded to is the same with the Pterotrachea coronata of 

 ForaskceL 



Although these genera have been constituted into a 

 separate order, it is probable that in a natural arrange- 

 ment they might more conveniently be placed with the 

 Pectinibranchiae. The position of the heart is indeed 

 singular ; but as this organ varies greatly in its form 

 and position in different genera, the characters which it 

 furnishes are consequently of doubtful value. The na. 

 ture of their hermaphroditism has never been satisfac- 

 torily investigated. 



