8 Essay introductory to Geology. 



proximation to the characters of tlie inferior fossiliferous beds on the one 

 Land, and to the superior tertiaries on the other. 



In these beds we have equally various aggregations of madreporal corals, 

 and althout^h many of the older forms of the lower groupe have disappeared 

 in the echinodermata, several varieties of echinus occur abundantly. Some 

 species of asterias and comatuia also appear; and a new division of cri- 

 noidea the articulata commences. It exhibits several genera and species, 

 most of which are confined to this geological groupe ; but one of the 

 genera, the pentacrinite, still exists ; and indeed the very recent species 

 differs but slightly from that of the lias. Among bivalves, the brachiopodal 

 class presents a large variety of species of terebratula, some of the older 

 forms of the lower groupe, e. g. the productas, still mark the rcagnesian 

 limestone, and the spiriferac extend as far as the lias. Of the other 

 bivalves, the genera are as numerous as those of the present day, with 

 which they nearly agree ;* but the species are always distinct. To the 

 univalves the same remark will partly apply, but we must modify it by 

 observing, that the genera of the division buccinoidea are few and rare in 

 the lower secondary beds, but suddenly develope themselves in great 

 numbers in the subcretaceous sands, which in this respect present an ap- 

 proximation to the tertiary character or superincumbent groupe. 



The cephalopoda exhibit in this groupe the family of belemnites, which is 

 peculiar to it and universal throughout all its members ; the ammonites are 

 also very abundant and characteristic, for though they have already appeared 

 in the lower groupe, still they are there rare, and they do not extend into 

 the tertiary groupe. Tliere are also some very remarkable genera, or rather 

 families of this order, peculiar to the cretaceous groupe, not being found 

 either above or below it; some of these, the turrilites, are distinguished by 

 the convolution of the chambered tube into the heliciform spiral, like a 

 corkscrew; in others this tube is only slightly curved at the extremity, 

 like a hook-hamites and lituites ; in others it is curved at both extremities, 

 scaphites. 



Marine Crustacea occur in the secondary groupe ; and here they no longer 

 exhibit those forms so remote from the actual types as in the trilobites of 

 the lower groupe, the existing genera astacus and paguras, (cray fish and 

 hermit crab,) are sufficiently marked, though the species probably differ; 

 they are found in the lias, in the lithographic calcareous slate of Solenhofen, 

 (belonging to the upper oolites) and in the chalk. 



Of the vertebrated fish, we find only the two earliest orders, (if we may 



* We should observe that the genus grj'phsea, which is exceedingly abundant 

 throughout this period, having twenty species, has only one recent, and even this is 

 very doubtful ; neither does it occur fossil above the chalk. Exactly the same re- 

 mark will apply to the genus trigoniic, of which twenty-one fossil and only one recent 

 species is known, and which is likewise wanting from the tertiary order of geologi- 

 cal formations. 



