96 STUDIES IN EVOLUTION 



Sea " (Schuchert^). The super-family Strophomenacea of 

 this order is the longest lived, and excelled in amount of 

 specific differentiation, there being 608 species in North 

 America alone (Schuchert). In this super-family the early 

 families and genera were without spines, it being only when 

 Chonetes is reached that the first spines are found in the 

 order. In this genus they are along the hinge and seem to 

 make up for the weak and obsolescent pedicle. Greater 

 spine growth occurs in the genera Productella and Productus, 

 where, in extreme cases, the surfaces of both valves are 

 thickly studded. During the Carboniferous the spiny Pro- 

 ducti attained their maximum both in number, length of 

 spines, and in individual size, for here occur the largest 

 species of all Brachiopoda. This was the climax. The 

 Permian genera are chiefly degenerate forms (Aulosteges, 

 iStrophalosia), and with the close of the Paleozoic the family 

 Productidse became extinct. The order Protremata, to which 

 this family belongs, likewise underwent a rapid decline, and 

 only two simple types continued on into the Mesozoic, while 

 but one declining representative is living at the present time. 



Among the Ammonoidea the chief spiny forms are those 

 occurring just before the final extinction of the group and 

 representing the beginning of the decline of the order 

 (^Crioceras, Toxoceras, Ancyloceras^ Hamites, etc.). In the 

 Dinosaurian Reptiles the great horned forms, Triceratops, 

 Torosaurus,^^ etc., mark the extinction of the entire order. 

 The great horned mammals of the Eocene, the Dinocerata, 

 have left no descendants, and the giant Brontotheriidge, after 

 undergoing various horn modifications through the Miocene, 

 continued no further. 



It is not desirable, however, to convey the impression that 

 the spines or horns are alone responsible for this wholesale 

 extinction. It has been shown that they are undoubtedly 

 often an expression of extreme specialization, and generally 

 they represent the limits to which superficial structures may 

 be differentiated. Although there may be other expressions 

 for similar conditions, yet the presence of spines is one, if not 



