REPORT ON THE POLYPLACOPHORA. 



13 



I have introduced this species for the sake of comparison with the foregoing, as they 

 are undoubtedly allied forms. There are several other species of Leptochiton which it is 

 unnecessary to refer to here. 



The following table has been drawn up to facilitate a comparison between the fore- 

 going species : — 



Surface of Shell. 



Arrangement of Tubercles. 



Jugum. 



Pleura. 



Lateral Areas. 



1. 



2. 

 3. 

 4. 



5. 



Smooth (?) 



» (?) 

 Smooth 



Striated 



Close 



More widely 



Close 



More widely and 



rather small 



Quincunx 



Fairly regular 



Irregular 



None 



Even, in no pattern. 



In series (?) 

 In series 

 Irregular 



None 



Irregular 



Irregular 



Smaller size, and more distant 



Minute and scattered 



From this table it will be seen that the first three species belong to the same group of 

 forms. Dall hesitated about separating his species from Leptochiton alveolus, and he, 

 Boo°- Watson, and myself, at first referred the two latter to the same species ; and it is only 

 after repeated and careful scrutiny that I have finally decided to consider them as distinct. 



We are still in the analytical stage with regard to this group, and it is perhaps advis- 

 able, at all events for the present, rather to emphasise any distinctions, and with this 

 intent I have described my specimens with as much minuteness as possible. Unfortunately 

 Dall has given us no figure of Leptochiton belknapi, but I have little doubt that the 

 Challenger specimens from the Philippines are examples, or, at most, varieties of that 



species. 



Genus LTemiarthrum, Carpenter, MS. 



Hemiarthrum, Carpenter M.S., Dall, Bull. U.S. Nat. Mus., 1876, vol. iii. p. 44. 

 " Valvge terminales laminatse, baud articulatse ; laminae laterales obsoletee ; zona lanu- 

 ginosa, porifera ; branchiae posticee." 



LTemiarthrum setulosum, Carpenter MS. (PL I. fig. 4 ; PI. II. figs. 4a-4Z). 



Hemiarthrum setulosum (Carpenter MS.), Dall, Bull. U.S. Nat. Mus., 1876, vol. iii. p. 44. 

 Dall, Proc. U.S. Nat. Mus., 1878, p. 314. 



