ON THE OPERCULA OF THE FAMILY STREPOMATID^. 



BY W. D. HARTMAN, M. D. 



Having enjoyed facilities for the comparison of large quanti- 

 ties of operculfe of the various genera of the family StrepomatidjB 

 undertaken chiefly with a view to discover differences that might 

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

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

 luscous animals. In speaking of the operculfe df 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 mar<^in' of 

 the operculigerous lobe, situated on the posterior part of the 

 toot of the animal. These grooves are successively developed 

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

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

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

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

 spiral laminse 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 labrum 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 root. Each plate is thicker on the columellar margin and tapers 



