LIFE. 495 



modern type ; 125 out of the 140 Cretaceous species were of this 

 family. Again, of the few Palaeozoic genera that hold over into the 

 Mesozoic, several die out before its close. This is true of Spirifer, 

 Spirigera, Cyrtia, Leptcena. The last species of the Spirifer and Lep- 

 tcena families lived in the Lias, and the Mesozoic representatives 

 of these prolific genera are small compared with the earlier, — those 

 of Leptcena hardly larger than an apple-seed (fig. 696 a). 



Spirifer (and its allied genera) had 200 Palaeozoic species, 12 to 15 Triassic, 

 9 Jurassic ; Leptsena, and the closely-allied Strophomena, lb to 100 Palaeozoic, and 

 only 5 of the former genus occur in the Lias, with which the type ends. Rhijn- 

 chonella is one of the few Brachiopod genera having living species : there were 

 about 120 Palaeozoic species, 50 Jurassic, 30 Cretaceous, — indicating no diminu- 

 tion in relative number, considering the length of time for each era, though in 

 number of individuals under the species the diminution is marked. 



The decline of the Brachiopod type is further observed in the 

 fact that the Terebratula family, which is the prevailing one in 

 the Mesozoic, is low in grade, as shown in its simple internal struc- 

 ture and elongate form. The other Brachiopod groups cotempo- 

 raneous with it are nearly of the same inferior rank. Lingula, 

 which began in the Primordial, and still exists, is one of the lowest 

 types of the order. 



Decline of the Cephalopod types of Orthoceras, Goniatites, and Conularia. 

 — The simple Orthoceras, which began in the Potsdam period, had 

 its last representative in the Triassic. The genus Goniatites, of De- 

 vonian origin, also ended in the Triassic. The genus Conularia, 

 dating from the Lower Silurian, became extinct in the Jurassic. 



Disappearance of vertebrate-tailed Ganoids. — Of Ganoids with verte- 

 brated tails — the only kind in the Palaeozoic — a few occur in the 

 Triassic, and these are the last fossil species of the kind. 



Decline of the type of Ces-traciont Sharks. — This decline commenced 

 in the Palaeozoic and continues through the Mesozoic. It is now 

 nearly an extinct family. 



2. Progress in Mesozoic Features. 



Culmination of the type of Cycads. — The plants of the Mesozoic were 

 mainly Ferns, Conifers, and Cycads. The ferns belong to a type — 

 that of Acrogens — which had passed its time of culmination in the 

 Palaeozoic. The Conifers were to have their fullest display in a 

 later era. The Cycads are eminently Mesozoic. They appear to 

 have begun near the close of the Carboniferous age, and passed 

 their climax in the Jurassic period. 



In the Mesozoic era, 10 or 12 Triassic species of Cycads have been recog- 



