On the Form, Growth, and Construction of Shells. [249 



FlG. 12. Unio tumidus — River Lea. u, the umbo of 

 the right valve ; I, the ligament ; f, the foot ; the 

 arrows indicates the respiratory currents. 



carbonic acid in solution, or dilute sulphuric acid from the de- 

 composition of iron pyrites. But the action is especially mani- 

 fested in those stagnant waters where the first probe of the 

 collecting-rod disengages from the mud an abundant stream of 

 bubbles of sulphuretted hydrogen. In such situations the spiral 

 shells — for example, Bithynia — have lost the ends of their spires, 

 and the discoidal shells, like Planorbis, have been found with 

 a small hole caused by the dissolution of the inner whorls. 



The great and ponderous mussels of the American rivers, 

 and even the fresh- 

 water unios and ano- 

 dons of our own 

 streams (see Wood- 

 cut, Fig. 12) are often 

 externally eroded, 

 andthe cause hasbeen 

 the subject of much 

 speculation. The 



umbo is the part first 

 formed, and conse- 

 quently that where 

 the epidermis is thin- 

 nest and has been longest exposed to the action of the ele- 

 ments, and it is this portion of the shell which is most corroded. 

 The epidermis of Trigonia is formed of a layer of nucleated 

 cells, difficult to detect, however, in the state in which they 

 are usually sent home {i.e., well cleaned). 



Dr. Bowerbank wrote a paper to show that the epidermis 

 was a very highly-organized structure, not only endowed with 

 vitality like our cuticle, but furnished with vessels appropriate 

 for and capable of repairing any injury it might sustain {Micr. 

 Soc, 1843). These observations, however, have not been 

 confirmed. 



Shell-structure. — Shells are composed of carbonate of lime, 

 with an animal basis. There is no appreciable amount of 

 phosphoric acid, so that shells or shell-sand have no chemical 

 value as fertilizers. The French chemist, M. E. Fremy, found 

 that the composition of shell was carbonate of lime, with 

 not more than one or two per cent, of phosphate of lime. 

 When treated with hydrochloric acid, there was left a brilliant 

 felt-like organic residue, not soluble in hot water, like horn or 

 gelatine, and insoluble in alcohol and ether. 



This substance, which is isomeric with osseine in bone, he 

 named conchioline. It sometimes exists in such large propor- 

 tions that, like the animal basis of human bone, the lime may 

 be dissolved out with weak acid, and the film of conchioline 

 will remain with its peculiarities of structure. 



