340 MISCELLANEOUS PAPERS. 



creases in age or in growth. After a while the depth of the boriug is 

 equal to the thickuess of the shell iu which it has been made, and the 

 occupant of the latter, in order to keep his own shell intact and main- 

 tain the integrity of his own domicile, commences depositing layer upon 

 layer of nacreous or porcellaneous matter, as the case may be. In 

 keeping pace with the continued encroachments of the domiciliary 

 squatter upon the outside, this deposit finally becomes a more or less 

 conspicuous protuberance. 



In one of the mother-of pearl shells which illustrate this paper (Plate 

 I, which exhibits, as do all the figures, the inner side of the shell), the 

 exterior has been perforated in both directions; that is to say, /rons- 

 versely, or at right angles to the (jrowthlayers of the shell also heticecn 

 the growth-layers, the first class of perforations leading to the sei>arate 

 rounded nodules and the latter to the elongated form. If the reader 

 had the shell iu his haud, so that he could turn it over and examine 

 the exterior perforations, he would at once perceive that the site of each 

 of the nodules or tubercles exactly corresponds with the site of an ex- 

 terior perforation or burrow. 



In the Maliotis shell, the domiciliares are a species of pholad {Fenitella 

 parva Tryon), and the site of each mixy be seen on the outside of the 

 shell and the corresponding nodosity on the inside. 



Plate I represents an adult abalone or ear-shell, Ilaliotis rtifescens 

 Swainson (Mus. No. 74877), a Californian species. The figures 1, 2, and 

 3 indicate the rounded elevations of nodules caused by Fenitella^ and 

 figures 4, 5, and G show the ends or edges of the pholad shells, the sur- 

 face of the ear-shell having been boied through or otherwise removed. 



Sometimes these nodules or tubercles are due to some foreign inor- 

 ganic matter, a particle getting iu between the mantle of the mollusk 

 and the inner surface of its shell. In such cases it is, we may say, at 

 once plastered over, and thus fixed upon the surface of the valve. 



Free concretions, i. c, unattached or nonadherent nodules, are, as is 

 well understood, caused by some particle, organic or inorganic, becom- 

 ing in some way lodged exclusively in the soft parts of the body of the 

 mollusk, and so far awa}- from the surface of the shell as not to admit 

 of its being cemented to it. 



No doubt many of the moUusca, both gastropod and lamellibranch, 

 contain or are inhabited by true parasites. In certain species of fresh- 

 water mussels {Anodonta cygnea of Europe, and Anodonta flnviatilis of 

 America) a species of water mite (Atax) has been detected, and some- 

 times Filaria (thread worms) aiul other forms occur. 



CRUSTACEAN PARASITES. 



A small species of crab {Pinnotheres), an epicurean no doubt, finds 

 a salubrious habitation iu the common oyster, but parasites of any con- 

 siderable size a])pear to be rather rare. 



Besides Pinnotheres ostrcum Say, the name of the species above referred 



