iNcisuRA (scissitrella) lytteltonensis. 3 



Pelseueer (12), in his well-known memoir on the morphology 

 of primitive mollusca, gives seven figures of sections of 

 S. costata and two of Incisura lytteltonensis in addi- 

 tion to three figures of the external features of the latter 

 species. The description he gives in the text is concise, and 

 furnishes a good general idea of the anatomy of the family ; 

 but he does not give sufficient detail to enable one to make 

 a critical examination of its systematic position. Hence, 

 having sufiicient material at ray disposal, I have thought it 

 worth while to make a thorough study of the anatomy of 

 Incisura lytteltonensis. 



Incisura, as Mr. Hedley states in a letter accompanying the 

 specimens, is found on the seaweed Cystophora in rock-pools 

 in Lyttleton Hai'bour, where it is associated with Rissoina, 

 Cantharides, and Gibbula. It may be inferred from its shape 

 and structure that it is semi-sessile in habit, but it is not 

 attached to one spot like a limpet. On the contrary, it is 

 fairly active, and one of the specimens was observed to crawl 

 for a distance of nearly half an inch in the space of a quarter 

 of an hour. When alive it is of a pink colour, and this tinge 

 is sometimes preserved in the shell. The length of the 

 animal, when contracted in spirit, is about 1 mm. 



External features. — These have been correctly if somewhat 

 diagrammatically figured by Pelseneer. A three-quarter 

 ventral view of the animal is given in fig. 1. Attention may 

 be called to the following points : The visceral spire is 

 attenuated and much reduced, its coiled apex containing only 

 some lobes of the liver and, in some specimens, a portion of 

 the gonad. The last whorl is greatly expanded laterally, 

 compressed dorso-ventrally, and contains all the important 

 organs of the body. The snout is moderately long, termina- 

 ting in a trumpet-shaped expansion, on the ventral side of 

 which is the mouth. The mantle is large, and in contracted 

 specimens completely covers the head and the greater part of 

 the snout. The mantle slit, corresponding to the la,bral 

 incision in the shell, is short, and situated nearly opposite the 

 right eye; its margins are furnished with short digitiform 



