146 DR. H. LYSTER JAMESON ON THE [Mar. 4, 



shell is obvious in such forms as Margaritifera, Pinna, and 

 Tridacna, where the mantle-margin is freely retractile. 



The evidence adduced in support of the alternate theory of 



■growth by intuvssusception, originated by Mery (1712), revived by 



von Nathusius (1877 & 1898), and supported by Felix Miiller 



(1885), is not convincing. The facts supposed to lend weight to 



this hypothesis are quite explicable on the apposition theory. 



Pearls, Bliste?'s, a/nd Conc7'etions. 



As some confusion exists as to the exact connotation of the 

 Avord pearl, I propose to adopt in this paper the terms " peail," 

 "blister," and "free concretion" for three different kinds of 

 structvires that occur in molluscs. 



Pearls. — A pearl consists of one or more layers of shell-substance 

 {i. e., conchyolin in which the crystals of inorganic matter are 

 disposed in the same manner as in the shell), enclosing a central 

 nucleus, and formed in a closed sac embedded in the tissues. 

 This sac is composed of epithelial cells similar to those that form 

 the outer mantle- epidermis. This sac is first formed around a 

 parasite, which probably exercises a specific stimulation. 



In Mytilus edidis and many other forms this parasite is a larval 

 Trematode, but it is pi'obable that certain other parasites can 

 stimulate some molluscs to form such sacs. The parasite does not 

 necessarily become the nucleus of the pearl, but may escape from 

 the sac before calcification. 



Any of the substances which form the difierent parts of the shell 

 may be represented in a pearl. Thus we have nacreous pearls, 

 prismatic pearls, the periostracum pearls of Modiola inodiolus 

 formed in the mantle- margin, pearls a pai-t of which may be 

 formed of the transparent striated substance which characterizes 

 the attachment of the muscles to the shell, and pearls formed 

 entirely or in part of the substance of the hinge-ligament. Large 

 brown leathery hinge-peai-ls are occasionally found in Torres 

 Straits in Margaritifera maxima Jameson. 



A pearl may become secondarily fused to or embedded in the 

 substance of the shell by the absorption of intervening tissues 

 (text-fig. 22). These pearls are sometimes spoken of as attached 

 peails. Similarly two or more pearls may become fused together, 

 forming double oi- compound pearls, of which a notable example 

 is the celebrated " Southern Cross." 



The various substances, when two or moie are present, are 

 not always arranged exactly in the leverse oider of the layers 

 of the shell, as sometimes stated. We may, indeed, have several 

 alternations of nacre and prismatic substance, or of the latter 

 and conchyolin. The kinds of shell-substance entering into 

 the composition of a pearl are determined by the position of the 

 latter. De Yillepoix (1892) has shown that different parts of the 

 mantle- epithelium are concerned severally in the formation of 

 periostracum, prismatic and nacreous substance. Obviovisly the 



