226 Analyses of Books, I^EP-r^ 



families of this section of trachelipoda in any secondary bed> 

 excepting the denuded tracts of green sand in Devonshire ; and 

 there, perforations exactly similar to those which abound among 

 tertiary and recent shells are also of frequent occurrence, 

 although I have never met with any such perforation in any 

 other secondary formation, nor even in any of those regular beds 

 of green sand, which actually underlie the chalk in other coun- 

 ties. I am not enough of a geologist to decide, as to whether 

 any admixture of secondary and tertiary fossils may possibly 

 have taken place when these denudations were made, but I can 

 in no other way account for the fact, that all the species which 

 have been perforated, as well as the carnivorous trachelipodes 

 themselves, are nearly similar to those of the London clay ; and 

 I have never been able to find any such perforation in either of 

 those species which are found in the more regular beds of green 

 sand, and which are sometimes mixed with them. These per- 

 forations may be readily distinguished from those more oblique 

 and lateral burrowings which are often found in secondary fos- 

 sils, and are always conveniently formed for suction by being 

 broadest in the outer surface, and go straight through that part 

 of the shell which is immediately over the animal ; whereas in the 

 latter the holes are cylindrical, and much more resemble the 

 indiscriminate burrowings which are common in recent oyster 

 shells. 



" In my former Letter, which the Royal Society has done me 

 the honour to publish in the Philosophical Transactions of last 

 year, I have pointed out some of the changes which took place 

 immediately after the chalk formation was completed ; and of 

 the British strata it may be further observed, that it is only in 

 the tertiary beds that any traces of the cirrhipeda, or of any of 

 the families of naked mollusca have been found. The beak, 

 which has been figured by Blumenbach, and which among the 

 fossils of the has is mentioned by Conybeare and PhiUips as the 

 beak of a sepia, belonged, as I think, unquestionably, to the 

 cephalopode animal of an ammonite ; and it sufficiently resem- 

 bles the lower mandible of the parrot-like beak which Rumphius 

 has described of nautilus pompilius. As might be expected, if 

 these mandibles, or rather casts of mandibles, belong to the 

 ammonites, they differ generically in shape from those of every 

 living genus of cephalopoda which has been figured or 

 described, and I have found them in all those beds ; and, so far 

 as 1 can ascertain, they have been discovered in those beds only 

 of the lias, lower oolite, and chalk, which contain the larger 

 ammonites. From the greater tenuity of these beaks in the 

 smaller species, they may probably have yielded to pressure, 

 and decay before the mud which filled them had become sufli- 

 ciently hard to retain their shapes ; and as the lower mandibles 

 of the cephalopoda are always much larger and thicker than the 



