354: Dr. Johnston's Contributions to the British Fauna. 



However distinct the specific characters may at first sight ap- 

 pear, yet I entertain no doubt in my own mind of its identity 

 with our P. imbricata. The scales are the same both in shape 

 and numl)er; nor indeed is there a character to distinguish it 

 except the dorsal fascia ; a character, probably dependent on 

 local causes, and certainly not of itself sufficient to constitute a 

 species. It would appear then that of this genus we have only 

 four well ascertained species, viz. the scabra and imbricata de- 

 scribed above, aud the cluva and viridis of Montagu. The others 

 said to be British, must be re-examined before they can take an 

 undisputed place in our Fauna. 



The above descriptions, I wish to remark, are purely specific, 

 the characters applicable to the genus being purposely omitted. 

 'J'hese characters as detailed by Lamarck from Savigny, I must 

 confess I do not understand. They are as follows : " Tentacula 

 simplicia, conica, proboscidis orificium coronantia. Maxillce cor- 

 neje. Antennce quinque ; interdum imparl nulla. Ociili quatuor. 

 Sqiiamoi dorsales." The antennae are defined to be jointed fila- 

 ments inserted on the head. Such organs I have never been 

 able to perceive, and I therefore believe them to have no ex- 

 istence. Nor can it with any propriety be said that the tentacula 

 encircle the orifice of the proboscis, for they are in reality placed 

 at the sides. The following appears to be a correct description 

 of the parts of the head in the species observed by me. 



The mouth is inferior, and from it the animal evolves, in the 

 same manner as the snail moves its tentacula, a thick cylindrical 

 proboscis armed with four brown hooked horny mandibles. Above 

 it is the head, a square corneous plate emarginate in front, and 

 bearing four small black eyes placed in a square. From its an- 

 terior margin two very short tentacula arise. These from their 

 situation must be the antenna; of Savigny, a denomination to which 

 I feel disposed <o object, since they have nothing in their struc- 

 ture, nor probably in their function, analogous to the antennae of 

 insects. Between them there is an acicular-polnted teutaculum 

 similar in all respects io the tentacular filaments, (acicules of 

 Savigny,) which garnish the sides ; and exterior to them, and a 

 little beneath them, there is, on each side, a strong conical ten- 



