450 NOTES ON PROSOBRANCHIATA, 



the conclusions arrived at. So far as our knowledge at present 

 goes the species of the " tritonis ''-group (post, p. 474) have a 

 dentition somewhat different from that of other groups, and 

 the so-called subgenus Guttumium has been stated by 

 Fischer (12, 655) to have an operculum distinct from that of 

 the rest. I would, however, draw attention to the fact that only 

 about half-a-dozen radulse have been figured, and of these few 

 that of L. femorale, Linn., is intermediate between those of L. 

 tritonis, Linn., as the one extreme, and L. cutaceum, Linn., as the 

 other. Hitherto there have been known three apparently distinct 

 types of protoconchs — those typified by L. riitihim, Menke 

 (PL xvii., fig. 21), L. cormUum, Perry (PI. xvii., fig. 10), and L. 

 woodsi, Tate (PL xvii., fig. 1). These are later shown to be 

 extreme forms of one generic type. It is more than probable that 

 when the subject is systematically investigated the two types of 

 dentition will suffer the same fate, and all the intermediate forms 

 will be forthcoming. 



The fact that Guttumium has an operculum differing from that 

 of other groups is not necessarily of much importance; since there 

 is much variation in the form of the protoconch and dentition 

 (and these variations of little note), a certain amount of variety 

 in the operculum is also to be expected. 



As already stated, my anatomical investigations have not been 

 extensive; the cause of this has been the difficulty of obtaining 

 material. Having in view the proposal to use four generic names 

 for this group, I have thought it as well to publish this essay in 

 its present incomplete form, and to follow it with an anatomical 

 supplement as soon as sufficient material can be obtained. 



Concho logical Evidence (Recent Species). 



The genus has hitherto been divided on the form of the shell 

 only; with what diversity of opinion is shown in the next part 

 of my paper. In the present part I propose to use that evidence 

 with, I submit, rather conclusive results, to point out the use- 

 lessness of such divisions as nomenclatural entities. For want of 

 better I have had to use such phrases as "nearest ally" and " nearly 



