of the Piedmontese Coast. 175 



(as in the Cowries) covered by the mantle of the animal, and the 

 consequent deposition of shelly matter, so as to conceal the 

 spire, 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 Fissurella. 



The present 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 temperature 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 Mediterranean waters average about 

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

 the xA.tlantic Ocean. 



The greatest specific variation between the British Testacea 

 and those of the Mediterranean occurs, as might have been 

 expected from the difi'erence of latitude and temperature, in 

 the denizens of the littoral and laminarian zones ; particularly 

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

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

 seem to be represented by their analogues ; as Mytilus edulis, 

 Chiton cinereus, Patella vulgata, Trochus lineatus, Buccinum' 

 undatum and Fusus Islandicus of our own coasts are respectively 

 replaced in the Mediterranean by Mytilus minimus, Chiton Sicu- 

 lus, Patella scutellaris, Trochus fragarioides, Murex trunculus and 

 Fusus comeus. 



It is remarkable that examples of the same species from the 

 Mediterranean are smaller than those found in the British seas. 

 Tellina balaustina, Jeffreysia 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. MacAndrew, each 

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

 and Azores, possesses 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 



