11 



small specimen at the Quarantine Station, Sydney, in 1918, 

 I at first thought it a different species owing to its wide 

 diverging dorsal area, but at last noted that it was only a 

 juvenile character. 



Mr. Gabriel's small shell from Port Philip, about 7J mm., 

 which I understand has been recorded as A. ruhrostratus, 

 Torr, is similar to the juvenile costatus in having small spicules 

 clothing the girdle and in having the broad dorsal area of 

 the juvenile, but it differs in not having coarse pustulose 

 ribbing behind the mucro and in the pustules of the diagonal 

 ribs being small; in these two respects only does it resemble 

 ruhrostratus and speciosus. I believe that had it attained a 

 larger growth it would have been quite similar to typical 

 costatus in these respects. It may be that in Victoria a race 

 of costatus is living that attains the senile characters at a later 

 age than is common to that species. 



Throughout this paper all measurements given are of dry 

 specimens. I have two specimens collected by myself in Gulf 

 St. Vincent, measuring 8 and 18 mm. long, and I have com- 

 pared them with Dr. Torr's type and co-type of A. ruhro- 

 stratus, and find them con-specific; the girdle is covered with 

 coarse white spicules, similar in size to those clothing the 

 girdle of A. speciosus, H. Adams, but appear less irregular in 

 their attachments and show between the valves in a much less 

 degree; the dorsal area is similar in the two, except that the 

 pinnatifid character is more continuous in richrostratus ; 

 neither have ribs behind the mucro, although one of Dr. 

 Torr's latter shows throe waves or undulations corresponding 

 with the posterior ribbing of costatus, nor have either the 

 coarse pustules of that species in the diagonal ribs. The 

 width of the gfirdle of ruhrostratus is less than that of 

 speciosus, and the beak of speciosus is less pronounced, but 

 when it is considered that the whole of our specimens of 

 ruhrostratus are smaller than the smallest of our specimens 

 of the other species, is it not possible that the slight differences 

 enumerated may be due to the juvenility, and that ruhro- 

 stratus is really the young of A. speciosus. 



In conclusion. — As before mentioned, I have been much 

 indebted to Dr. Torr for the opportunity of examining speci- 

 mens in his collection, but until a much larger series of all 

 the species, in all stages of growth, is available, I do not 

 like to make a final decision, but I am inclined to think that 

 ultimately we shall be able to recognize A . costatus as the 

 dominant species, with two sub-species — gahrieli, Ashby, from 

 Queensland, and suh-viridis, Torr, from Western Australia — 

 A. costatus, sub-species, being found in New South Wales, 

 Victoria, Tasmania, and South Australia; A. speciosus, 



