C. E. Beecher — Origin and Significance of Spines. 351 



radicles, which, though differing in internal characters, still 

 retain a primitive simplicity in superficial features. The early 

 stages of ontogeny of any form should agree with the radical 

 stock, and, as already noted, these stages are simple. Hyatt 34 

 says on this point : " the evidence is very strong that there is 

 a limit to the progressive complications which may take place 

 in any type, beyond which it can only proceed by reversing 

 the process, and retrograding. At the same time, however, 

 the evidence is equally strong that there are such things as 

 types which remain comparatively simple, or do not progress 

 to the same degree as others of their own group. Among 

 Nautiloidea and Ammonoidea these are the radicle or generator 

 types. No case has yet been found of a highly complicated, 

 specialized type, with a long line of descendants traceable to it 

 as the radicle, except the progressive ; and all our examples of 

 radicles are taken from lower, simpler forms ; and these radicle 

 types are longer-lived, more persistent and less changeable in 

 time than their descendants." 



A few examples will now be taken from the life histories of 

 large groups. In the Brachiopods, the order Protremata, con- 

 taining most of the spinose forms, has 4 genera and 22 species 

 in the Cambrian of America, 20 genera and 173 species in the 

 Ordovician, and 30 genera in the Silurian. "Then began a 

 steady decline, with extinction in the Carboniferous of North 

 America. In the Triassic of Europe this order is sparingly 

 represented by small species, and is there essentially restricted 

 to the family Thecidiidae, which continues to have living repre- 

 sentatives in the Mediterranean Sea" (Schuchert 64 ). The 

 superfamily 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 

 superfamily 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 ped- 

 icle. Greater spine growth occurs in the genera Productella 

 and JProductus, where, in extreme cases, the surfaces of both 

 valves are thickly studded. During the Carboniferous, the 

 spiny Productii attained their maximum both in number, 

 length of spines, and in individual size, for here occur the 

 largest species of all Brachiopods. This was the climax. The 

 Permian genera are chiefly degenerate forms (Aidosteges, 

 Strophalosia), 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 



