Mr. W, Clark on the Genus Truncatella. 5 



respiratory systems are in respect of these animals on the limits 

 of their respective boundaries^ and form the transitions from one 

 to the other. Both the Pectinibranchiates escaped from the water 

 as quickly or quicker than the Pulmonifera, but all the former, 

 whether littoral or from deep water, have the same propensity ; 

 therefore this fact is of little value ; yet, when strictly confined, 

 the Truncatellce continued lively for two days, whilst the Cono- 

 vuli became torpid in twelve hours : perhaps we may conclude 

 that these littoral Pectinibranchiata and the suh-littoral Pulmo- 

 nifera are to a certain extent amphibious. All these species were 

 taken mixed together in an estuary at Portland by Mr. Thomp- 

 son, who states that they are very rare and local. 



Truncatella Montagui, Lowe. 

 Ti'uncatella Montagui, Brit. Moll. 



Animal, when adult, occupying a yellow or whitish subcylin- 

 drical shell of four flattish volutions deeply divided, and fur- 

 nished with close- set somewhat irregular costellse; when young, 

 before the apex is truncated, of 6-8 smoother and more taper 

 gyrations ; the peristome in the completed shell is entire, having 

 the outer lip blunt and a little reflexed ; with respect to colour, 

 it is of the palest yellowish white, aspersed with very minute 

 flake-white points, particularly the foot, with a patch of pink on 

 the neck caused by the colour of the buccal corneous plates 

 showing through the pellucidity of the tissue. The mantle is 

 plain and even with the shell, but rather tumid at the margin : I 

 did not observe much approach to the mantellar collar of the 

 Helices. The I'ostrum is ridged or annulated, long, very broad, 

 flat, emarginate at the end, forming on each side a curved com- 

 pressed auricle, and cloven underneath vertically and slightly 

 crosially ; buccal apparatus reddish pink ; the tongue at its deeply 

 seated terminus displays a pair of white jaws ; it can be seen 

 through the oesophagus, and is accompanied on each side by a 

 linear streamer floating loose posteally. I am not certain whether 

 these narrow tape-like additions proceed from the buccal mem- 

 brane or tongue, — I think the latter, — or they may possibly be 

 salivai-y glands. 



The branchial plume is single, of an elongated kidney-shaped 

 figure, and has the usual constriction or sinus at the end nearest 

 to the heart ; it can be detected with high powers in sunlight, 

 through the body volution of pale, clear, thin shells ; all the rest 

 of the organs, including the single pale brown ganglion on each 

 side the oesophageal collar on its upper sin:face, may be easily 

 seen through the hyaline tissue of the neck and head. The neck 

 and rostrum from its corrugations can be protruded to an extra- 



