of the Piedmontcse Coast. 175 



(as ill tlic Cowries) covered by the mantle of the aiiiinal, and the 

 consequent dej)Osition of shelly matter, so as to conceal the 

 s])irc, the truncation or decollation of those whorls (as in Ccecum), 

 or the altered position of the branchial opening and subsequent 

 loss of the spire, as in FissurcUa. 



The ])resent distribution and existence of the same species of 

 marine Testacea, in many and widely separated parts of the 

 globe, may be in some measure accounted for by the equable 

 temperature which is usually maintained in the sea, independent 

 of climate, and by the want of solar influence beyond a limited 

 depth ; water being, as is well known, one of the worst con- 

 ductors of heat. Admiral Smyth states that there is a sensible 

 diminution between the surface-temperature and that obtained 

 at great depths in the Mediterranean, and which he roundly 

 estimates at 1° for every twenty fathoms in depth, except where 

 the agency of submarine currents may be at work, but that 

 below 180 fathoms to the greatest depths which he had ex- 

 plored, the tcmjicrature varied but little from 42° or 43° of the 

 Fahrenheit scale ; and he adds, that a comparison of his eight- 

 fathom observations on the mean temperature of that sea, led 

 him to consider that the jNIediterrancan waters average about 

 3° 5' of Fahrenheit more heat than that of the western part of 

 the Atlantic Ocean. 



The greatest specific variation between the British Testacea 

 and those of the Mediterranean occurs, as might have been 

 expected from the difference of latitude and temperature, in 

 the denizens of the littoral and laminarian zones ; particularly 

 in the genera Mijtilus, Chiton, Patella, Trochus, Buccinum, 

 Fuaus, and Murex. In each of those zones certain species 

 seem to be represented by their analogues ; as Mijtilus eduUs, 

 Chiton cinereus, Patella vulcjata, Trochus lineatus, Buccinum 

 undatum and Ficsus Islandicus of our own coasts arc respectively 

 replaced in the ^Mediterranean by Mytilus minimus, Chiton Sicu- 

 lics, Patella scutellaris, Trochus fracjarioides, Murex trunculus and 

 Fusus corneas. 



It is remarkable that examples of the same species from the 

 Mediterranean are smaller than those found in the British seas. 

 Tellina balaustina, Jeffreijsia diaphana and Rissoa pulcherrima 

 are instances of this. 



A much greater range of variation is found to exist in land 

 than in marine animals, owing to the more uniform temperature 

 of the sea and its coasts. According to Mr. MacAndrcw, each 

 of the islands which form the groups of the Canaries, Madeiras, 

 and Azores, })ossesses some species peculiar to itself; and every 

 British conchologist is aware of the very limited habitat which 

 some land and freshwater shells, as Helix Pisana, Assiminia 



