316 AMERICAN JOURNAL 



ON THE OPERCULA OF THE FAMILY STREPOMATID^. 



BY W. D. HARTMAN, M. D. 



Having enjoyed facilities for the comparison of large quanti- 

 ties of operculse of the various genera of the family Strepomatidaj, 

 undertaken chiefly with a view to discover diiferences that might 

 exist in this organ between allied species, I find it exhibiting as 

 a family trait a homogeneousness of structure not usual in mol- 

 luscous animals. In speaking of the operculse of this family 

 writers usually designate them as corneous. They probably 

 differ from true horn in the larger percentage of siliceous or cal- 

 careous matter entering into their composition, a condition 

 inferred from their brittleness and the ability with which they 

 resist corroding influences. The organ in question, upon which 

 so much of the safety of this helpless little mollusk depends, is 

 secreted from small grooves or folds in the columellar margin of 

 the operculigerous lobe, situated on the posterior part of the 

 foot of the animal. These grooves are successively developed 

 during the several stages of growth, and are more readily seen 

 in the foot of an alcoholic specimen of some of the larger species 

 from which the operculum has been forced off than in one perfectly 

 fresh. The operculum is composed of semi-concentric or paucis- 

 spiral laminae or plates, each of which (as the animal matures) is 

 more narrow and thicker next the columella and becomes wider 

 and thinner as they approach the junction of the la brum with 

 the body whirl. At every stage of growth the new lamina 

 (which is partly overlapped by the preceding one) describes a 

 larger arc of a circle, which imparts the paucis-spiral or fan- like 

 form to the adult operculum. The imbricated arrangement of the 

 lamina, visible to a certain extent by transmitted light under a low 

 power, is more strikingly developed after boiling the operculum 

 for a few moments in liquor potassa and moderate compression 

 between two pieces of glass. It will then be evident that this 

 organ is composed of semi-concentric plates, which are renewed 

 from the columellar margin of the proligerous lobe, each layer 

 being within and partly beneath the preceding one, like tile on 

 a roof. Each plate is thicker on the columellar margin and tapers 



