80 TRANSACTIONS OF THE [jAN. 11, 



Another interesting case is that of the Australian Ilyochama 

 anomioides, which attaches to different shells of varied and di- 

 verse surface patterns, as beautifully illustrated by specimens 

 in the American Museum of Natural History. The shell is nor- 

 mally marked b^^ prominent bifurcating radial ridges. One 

 specimen attached to a lamellibranch with prominent concen- 

 tric ridges shows these accurately assumed, while others copy 

 no less minutely the radial folds of their hosts. Perhaps the 

 most interesting example is that presented by a Myochama at- 

 tached to a shell with sharp pectinate radial ribs. The former 

 is symmetrically situated upon its host with its umbo directed 

 downward, as shown in Fig. 2. 



Fig. 2. Shell of a ? Trigonki with attached valve of 3Tyochama!'anomioides, 

 which has partially assumed the radial folds of its host. The broken lines 

 indicate the radial pectinate ridges of the host, the double full lines those 

 proper to the ilyochama, and the single full lines those assumed by the latter. 



It will be seen that medianly and anteriorly the Myochama 

 has completeh' assumed the radial ridges of the attached sur- 

 face, and its own radial folds have become thereby almost en- 

 tirely suppressed. These, however, reappear further down, 

 where the ridges of its host were less prominent. Such a sup- 

 pression does not occur where the assumed ridges are perpen- 

 dicular to the radial folds of the Ilyochama. 



Numerous other cases are reported among fossil lamelli- 

 branchs, and is very frequently to be observed in Ostrea. 



The phenomenon then appears to be a very common one 

 among attached shell, and is found to be widely distributed. Its 

 immediate cause, as explained by Graf, is very obvious. During 

 the growth of the shell the mantle edge became pressed into and 

 followed all the inequalities of the attached surface, thus pro- 

 ducing a mould upon which the lime salts were deposited and 

 consolidated. 



Reasoning from the advantage that would accrue to the Grepi- 

 dula from always fixing upon an active shell like Pecten, Dr. 



