15 



and I have much pleasure in naming it after him. The speci- 

 mens submitted to me were badly encrusted and, in parts, 

 eroded ; the true characters only became visible on disarticu- 

 lation and cleaning. The shape of the articulamentum both 

 in median and tail valves, the fact that the dorsal area is 

 usually concave, and the girdle densely spiculose, easily 

 separates this Acanthochiton from any other Australian 

 species. Tlie type belongs to the Queensland Museum. 



Specimen No. 2. Co- type. 



The second specimen, which I am calling the co-type, 

 differs from the type in that the centre of the dorsal area, in 

 the median valves, is slightly convex, becoming flat or slightly 

 concave before the pustulose sculpture is reached. Also the 

 pustulose character' of the sculpture is more limited in area, 

 anteriorly the pustules, which are very flat, become sub- 

 obsolete and confluent follov/ing the course of the growth-lines. 

 The CO- type remains in my collection. 



Acanthochiton retrojectus, Pilsbry, 



var. pustulosus, n. var. 



(A retrojectus, Pils., Naut., vii., p. 107, Jan., 1894; Proc. 

 Acad. Nat. Sci. of Phil., 1894.) 



Introduction. — In November, 1918, I collected a very 

 long series of A. retrojectus, Pilsbry, in the Quarantine Sta- 

 tion, Sydney Harbour. And early in 1919 the following notes 

 on this somewhat difficult species were written. At first one 

 concluded that there were at least two or three different species 

 represented in the series collected from the one spot. Many 

 had similar sculpture in the dorsal area to that of the other 

 areas, but in some this area was smooth, and, again, while 

 many had the regular, evenly-rounded, pustulose sculpture 

 described by Dr. Pilsbry, in others a large part of the shell 

 was ornamented with large tear-drop pustules. 



On fuller investigfation it was found that there was a 

 complete series of intermediate forms. All are similar in the 

 character and structure of the girdle, the shape and lamina- 

 tion and slitting of valves, the tail valve and position of 

 mucro; throughout these features seem consistent, and the 

 extreme divergence of sculpture does not, in my opinion, 

 warrant the separating of them into distinct species. Never- 

 theless, at the suggestion of my friend, Mr. W. L. May, I 

 propose a distinct varietal name for the form with coarse 

 pustules, calling it var. pustulosus . 



Girdle is densely covered with minute scales or short 

 blunt-ended spicules, these are mottled, the white ones often 



