538 POSITION OF PURPURA TRITONIFORMIS OF BLAINVILLE. 



By " Sinusigera-type " I mean an embryo bearing on its outer 

 lip the claw-like processes and consequent sinus so noticeable in 

 all the examples mentioned. I know that Dall"^ has proposed 

 the name Sinusigera for cancellated apices of the PleiLrotomidce; 

 but with all deference to one so much my senior I would suggest 

 that the term were better restricted to such as I mention above. 



The operculum, as Angas said, is purpuroid; the nucleus is 

 placed in the middle of the outer edge. The inner surface is 

 finely striated. 



The mollusc itself so closely resembles P. succincta that but for 

 the lingual ribbon it would be almost impossible for the dissector 

 to tell whether he were working on the young of that species or an 

 adult P. tritoniformis. The two dentitions are represented in 

 figs. 5-6 of Plate xxix. It will be observed that the dijfferenee 

 between these two radulse is one of degree rather than of kind. 



Summary. — In the preceding pages reasons are given for 

 removing P. tritoniforrnis from Urosalpinx and Comindla and 

 transferring it to Purpura. It remains to select a subgenus of 

 the latter for its reception. By the form of the shell Polytropa 

 might claim it, but the resemblance of the larval shell and of anato- 

 mical characters to P, succincta is so close that Trochia would 

 seem more appropriate. 



The names Adamsia and Agnewia consequently lapse into the 

 synonomy of Trochia. 



EXPLANATION OF PLATE XXIX. 



Fig. 1. — Young of Purpura succincta, Martyn; length 2'3mm. 



Fig. 2. — Young of Purpura tritoniforimis, Blainv.; length 3 mm. 



Fig. 3.— ,, „ ,, ,, ,, 5 ,, 



Fig. 4. — An embryo of the Sinusigera-type; length 2, breadth r3 mm. 



Fig. 5. — Dentition of Purpura tritoniformis. 



Fig. 6. — ,, ,, succincta. 



Fig. 7. — Operculum of Purpura tritoniformis (adult); 10'5 x 6*5 mm. 



Fig. 8. — Operculum of Purpura succincta (young); 18'12mm. 



Blake Moll. Vol. ii., p. 124. 



