112 CONCHOLOGY. 



the same time. In the Pteroceras, here figured, for 

 example, the young shell, instead of the elongated 



Young Shell. 



Adult Shell. 

 PTKROCKRAS SCORPIO. 



points, has open, waved prolongations, which, as the 

 organs they then covered increase in length, become 

 lengthened, and finally closed altogether, as the 

 animal, after attaining its maturity, finds it no longer 

 necessary to go on building, but then retreats into 

 its dwelling, and the very organs most serviceable to 

 complete it not being any longer in constant action, 



Murex tenuispina. 



rrae to grow, and, on the contrary, diminish in size, 

 and very frequently disappear altogether. In other 



genera, the Murex, &c., we see regularly-formed sets' 

 of spines, or foliations, at given intervals, round the 

 spiral form of the shell. Each of these, it is quite 

 clear, formed previous terminations, and became 

 successively closed as the increased size of the animal 

 required more room. These additions go on with 

 the greatest regularity, and correspond accurately in 

 all the species, with the exception of some few, which 

 may be considered sports of nature, and not generic 

 distinctions. By counting these periodical additions, 

 we come to the same conclusion regarding their age 

 as with the Wentletrap. We can indeed say that 

 such a shell is not an adult one, by counting the 

 number of varices, but it has not yet been determined 

 what period of time is necessary to complete the 

 standard measure. Helices, and their congeners, 

 never form a thickened lip until their full growth is 

 attained. Double-lipped shells have been described, 

 but they are extremely rare. We shall point out the 

 mode adopted by nature in forming the snail shell, 

 in which it will be seen, as before stated, that it ia 

 formed on the animal's body. It is equally certain, 

 though not so easily proved by observation, that those 

 shells which have claws, as they are termed, such as 

 the Pteroceras present, or foliations and spines, as the 

 Murices exhibit, must depend wholly on the form of 

 the skin of the mantle and its age, by means of which 

 these were formed, its lobes, prolongations of portions 

 of the mantle, the organs of respiration, the head, 

 oviduct, &c. ; each producing, by their habitual uses, 

 the spines, claws, canals, and other parts of the ex- 

 ternal covering of the animal. If the edges of the 

 mantle of the animal are even, the termination of the 

 shell is the same ; if waved, so is the external edge. 

 Where digitations of the mantle exist, they are 

 equally protected, and become, as we have stated, 

 claws or spines ; tubular in their early growth, but 

 rendered solid by repeated deposits of testaceous 

 matter, exuding from the mantle in certain propor- 

 tions, till the period of its full maturity. The animal 

 then declines ; its dwelling is sufficient for the pro- 

 tection of its body ; the wants of its life in seeking 

 food, or in moving from one spot to another, during 

 the completion of its growth, require an extension of 

 those parts, at first but slightly defined, and they are 

 not in the full exercise of their adult functions ; but 

 these extended organs in youth are much greater 

 in proportion, and frequently more numerous, than 

 they are in the stage of decrepitude, when they begin 

 to disappear, and the activity of their offices gra- 

 dually decline, being no longer required to complete 

 most of their destined operations. As a general 

 deduction, we repeat, it appears quite certain that 

 the spines, tubercles, and claws of shells, however 

 solid they may be met with, have all of them, at first, 

 been channelled or grooved, to defend certain ex- 

 posed organs in those which have the canal or slit 

 on the under side, and they are by far the most 

 numerous, having been produced by the digitations 

 of the mantle ; while those which have the slit on the 

 upper side, as in the Purpura and Venus Dione, 

 (Linnaeus) appear, on the contrary, to have been 

 produced by the concavity of an appendage to the 

 mantle, which projected on the outside. 



Nothing certain appears known of the age of 

 shells, though we have just remarked that certain 

 stages of growth are marked by signs of the shell 

 being then finished ; their growth probably in many 

 species, under favourable circumstances of climate 



