222 Zoological Society. 



II. PlACUNANOMIA COLON. 



Shell (upper valve) flat, with rather irregular, flat, radiating ribs ; 

 white, lower spotted ; upper valve with two separate scars ; the upper 

 one oblong, longitudinal, the lower much smaller, circular. 



Hab. ? 



Mr. Cuming's Collection (no. 10). Mr. Humphrey's Collection; 

 a single upper valve of a rather young shell. 



Here may be added the description of a new genus, intermediate 

 between this family and Placunidce. 



III. Hemiplacuna. 



Shell free ; valves orbicular, flat, external surface minutely laminar 

 and radiately striated, especially on the edge of the plates ; muscular 

 scar in each valve single, nearly central, circular ; the right valve flat, 

 with a large oblong, elevated transverse process for the cartilage, 

 having a very small concavity in the inner surface in front of the car- 

 tilaged process representing the sinus in Anomia ; the left valve rather 

 more convex, with an oblong transverse pit for the internal cartilage 

 under the umbo. 



Hemiplacuna, G. B. Soiverby, MSS. 



This shell has all the external characters of the flat species of Pla- 

 cxma, and has the same muscular impression ; but instead of having 

 the two linear diverging ridges and grooves to give attachment to the 

 cardinal cartilage, it has an oblong elevated process in the right valve, 

 and an oblong cavity in the left, exactly similar to those found in the 

 genus Anomia ; and on the inner surface of the right valve, just in 

 front of the base of the process which supports the cartilages, there 

 is a small shallow roundish pit with a short furrow towards the centre 

 of the shell, which is evidently a rudimentary representation of the 

 sinus found in the genus Anomia. This sinus is not visible on the 

 outer surface of the shell. 



This shell forms a most excellent passage between the genus Ano- 

 mia, or rather Placunanomia, and Placnna. It shows the gradual 

 change which takes place between the three genera. In Anomia 

 there are two muscles for the purpose of attaching itself to ma- 

 rine bodies, which form a plug which is free from the sinus of the 

 shell. 



In PIacnna)wmia there is only a single muscle to perform the same 

 office, but in the more typical species of this genus the plug itself is 

 affixed into the surface of the shell, forming, as it were, part of its 

 substance. In Hemiplacnna and Placnna there is no muscle or 

 plug for attachment, and the shells are free ; but in Hemipluciina 

 there is a rudimentary development of the sinus through which 

 the plug is emitted, and the ligament which connects the shell is 

 of the same form as that found in the genera Anomia and Placuna- 

 nomia. 



Mr. George B. Sowerby kindly showed me this shell, which he 

 purchased with a number of other fossil shells brought from the lied 

 Sea. He informed me that he intends to describe it at length, and 

 give it the name which I have with his permission here used. The 



