460 n. J. FLETJUE AND JIUKIHL M. GETTINGS. 



Montagu, into Gibbula obliquata^ Gmeliu, belonging to 

 the group of the Gibbuliutu. For the purposes of this paper 

 we call it T. obliquatus, GmeHu. I'he fault of the Pilsbry 

 sytem is that it is based too exclusively on concliological 

 differences, too little being known about detailed structure 

 to make an anatomical survey possible. In the definition of 

 the group Trochininee, however, we find that members of 

 this group do not possess jaws, and, as those structures are 

 present in T. crass us, Montagu, just as in T. obliquatus, 

 Gmelin, the unsatisfactoriness of the system is further 

 demonstrated. These jaws also show very interesting 

 adaptations in their structure and relations. 



They are paired chitiuous plates formed in the same way 

 as those of Haliotis (3). Here, as in Haliotis, the anterior 

 and downward-pointing ends of the jaw-plates do not lie 

 against the actual gut wall, but against mouthward-outgrowths 

 of the latter, so that the anterior edges of the plates are to 

 some extent free (tig. 1). The jaw-plates in these species 

 are much smaller and thinner than those of Haliotis and the 

 Docoglossa. In T. crass us, Mont., they are merely a pair 

 of dorso-lateral plates on projections of the wall of the buccal 

 cavity, continuous with one another across the median line. 

 The tissue beneath them, instead of being primarily muscu- 

 lar, as in Haliotis, has a structure (fig. 2) rather like that of 

 the superficial parts of the odontophore cartilages. It shows 

 muscle-fibres running in small strands through what is prac- 

 tically a mass of " cartilage " of loose texture. The contrasts 

 in structure point to functional differences between the jaws 

 of Troclius, and those of Haliotis and other Rhipidoglossa. 

 In Haliotis the jaws are distinctly lateral, and their strong 

 brush-like free edges help actively in the work of bringing 

 food fragments into the mouth. In Patella the two jaws arc 

 connected dorsally, and the edges form a strong arch which 

 leaves space enough beneath for the protrusion of the unusu- 

 ally broad and solid odontophore cushion ; the dorsal part of 

 the jaws is fairly flexible, and the lateral edges are thus able 

 to help in cutting food when the animal is browsing on a 



