1525.] Fhihsophical Transactions for 1824, Part I J, 227 



Upper Gftes, the non-appearance of any of the latter may be 

 accounted for in the same way. The sepiae are moreover fur- 

 nished with one of those thick dorsal plates which are commonly 

 called cuttle-fish bones, and most, if not all the other sepiadse, 

 contain an internal horny substance of the same nature, which 

 is generally at least as thick and durable as the mandibles ; and 

 if th6 fossil beaks of the secondary strata belonged to this 

 family, then, in all probabihty, some of the dorsal plates would 

 be found with them; but nothing ofthe kind has been discovered 

 in any older British stratum than the London clay. So far from 

 being able to detect any traces of the naked mollusca, 1 have 

 not been able to find, in the secondary strata, either of those 

 shells by which the animal is only partially covered, nor any of 

 those of the convoluta?, which necessarily change their shells at 

 different periods of their growth, and of which the animal must 

 therefore occasionally remain exposed, till a fresh coat of calca- 

 reous matter has been secreted. In my former Letter I have 

 stated, that all the marine spiiivalves of the secondary strata 

 belong to operculated genera, and these observations serve still 

 more strikingly to prove that, till the chalk deposits were com- 

 pleted, the mollusca, in our latitude, required a more perfect 

 protection either from their enemies, or from the surrounding 

 element, than afterwards became necessary. 



" The same gradual approximation towards recent shells, 

 which may be traced in the older strata, is also carried on. 

 through the tertiary formations, and the affinity, which is com- 

 plete with respect to orders in secondary beds above the lias, 

 becomes further extended, and every tertiary shell may be refer- 

 red to some existing genus ; but though the approximation has 

 proceeded thus far in the London clay, yet all its immensely 

 numerous species are now extinct ; and it is only in those upper- 

 rnost beds of crag, which lie between the London clay and our 

 present creation, that any fossil can be completely identified 

 with a living species : the shells which may be thus identified 

 are however mixed with many extinct species ; and though the 

 fossils ofthe crab appear generally to have belonged to a warmer 

 climate than ours, yet their character is much less tropical than 

 those of the London clay, and in every respect they all approach 

 nearer to the present inhabitants of the British coasts. 



" I have already observed, that the shells of unknown families 

 are confined to the beds below the lower oolite ; and in all the 

 upper formations a relationship is completed between fossil and 

 recent shells in the following regularly approximating series. In 

 the secondary strata above the lias as to jiatural orders, in the 

 London clay as to genera, and partially as to species- in the crag. 

 " These observations refer exclusively to the animals of the 

 9th, 10th, 11th, 12th, and 13th classes of invertebrata in La- 

 marck's arrangement; and whether the same sort of regularly 



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