172 CLASS I. TROPIOPODA. ORDER II. UN1MUSCULOSA. 



attaching itself, through a narrow longitudinal fissure ; it is moreover 

 confined within the aperture ; not free, as in Anomia. As the animal ad- 

 vances in growth this fissure is obliquely prolonged ; and the tendon of 

 adhesion, which is of a more bony character than that of Anomia, becomes 

 ossified in order to strengthen its power of attachment as the shell 

 enlarges. Thus the Placunanomia; are important as establishing a com- 

 plete affinity between two genera in which little had apparently existed. 



The shell of Placunanomia may be described as being irregular, nearly 

 equivalve, smooth, plaited round the edge, and somewhat glassy inside. 

 The hinge consists of two narrow ribs, unequal in length, converging at 

 the base, in one valve ; and in the other are two corresponding grooves 

 for the reception of the ligament which runs along the edge of the cardi- 

 nal ribs. The lower valve is longitudinally perforated near the hinge, 

 and the opening or fissure is entirely filled up by the ossification of the 

 tendon passing through it, a portion of which is also inserted between the 

 laminae of the shell. The impression of the chief adductor muscle is 

 nearly central in both valves, and there is also a mark in the upper 

 valve, which shows the internal attachment of the other extremity of the 

 tendon or organ of adhesion. 



Since the under-mentioned species has been described, one or two 

 others have been observed, though somewhat less characteristic. 



Example. 



PI. CXXIII. Fig. 1 and 2. 



Placunanomia Cumingii, Broderip, Proceedings Zool. Soc, 1832, p. 28. 

 Sowerbv, Genera of Shells, No. 38. 



ANOMIA, Linnseus. 



Testa insequivalvis, irregularis, affixa ; valva inferiore planiuscula prope 

 ad cardinem orbiculatim perforata, apertura parietibus reflexis ten- 



