scHuoHERT.l GENERAL DEVELOPMENT. 89 



either that evolution in pre-Cambrian eras was much slower than sub- 

 sequently or that the class had its oi-inin late in the Alyonkiaii. Cam- 

 brian brachiopods usually differ fundamentally from one another, and 

 do not appear to have been persistent, as but 4 of the 22 genera pass 

 into the Ordovician. Differentiation also appears to have been slow 

 during the Lower and Middle (Janibriau, but toward the close of this 

 system species begin to be more numerous and varied. In Middle Ordo- 

 vician times all the orders and superfamilies are well established 

 except Terebratulacea. The zenith of the class was attained in the 

 Silurian and Devonian eras, but decline began during late Devonian, 

 and steadily continued to the close of the Paleozoic. But 7 of the Car- 

 boniferous genera are known to have survived the break between the 

 Paleozoic and Mesozoic. During the latter time the spire-bearing 

 brachiopods pass out of existence, while the great Paleozoic super- 

 family Strophomeuacea is represented by a few small species of the 

 Thecidiidic, which continue to be represented up to the present time. 

 After the Cretaceous system the orders Atremata, Neotremata, and 

 Protremata are represented only by Lingula, Discina, Discinisca, 

 Crania, and Thecidium. The Terebratulida? may have had their inceft- 

 tion below the middle of the Ordovician, but are not a pronounced 

 Paleozoic group. However, in the Jurassic and Cretaceous systems 

 the rocks abound with the shells of this family, and from that time on 

 they are the chief representatives of the class. Lingula and Crania 

 are present in the Ordovician, and, as far as can be determined, have 

 persisted to the present time. 



Of the 49 families and subfamilies constituting the class, 43 became 

 differentiated in the Paleozoic, and of these 30 disappeared with it, 

 while but 13 continued from the Paleozoic into the Mesozoic. Of Paleo- 

 zoic families, are represented by living species, viz, Lingulid«, Dis 

 cinida^, Craniidie, Thecidiidie, Khynchonellidte, and Terebratulidie. 



Of the 327 genera now in use, 227 had their origin in Paleozoic seas, 

 or nearly 70 per cent of the entire class, and of this great number but 

 8 are positively known to pass into the Mesozoic, viz, Lingula, Orbicu- 

 loidea. Crania, Khynchonella, Spiriferina, Athyris, Terebratula, and 

 Hemiptychiua. Besides these, Streptorhynchus, Cyrtina, Retzia, Mar- 

 tinia, aiul Martiniopsis, are mentioned as occurring in the Triassic, but 

 these species probably in great part belong to other genera. 



The Atremata, which contains the oldest and the simplest forms 

 structurally, is represented by 29 genera, while the Neotremata and 

 Protremata have 30 and 89, respectively. Telotremata is the last 

 order to appear, and has by far the greatest number of genera, 179. 



The chronogenetic history of brachiopods shows that the four orders 

 begin with smooth shells, and that subsequently various kinds of sur- 

 face ornamentation are developed or disappear witli varying degrees of 

 rapidity. The ontogeny of strongly plicated and lamellose shells, wher- 

 ever observed, begins with smooth shells. All new surface characters 



