88 SYNOPSIS OF AMERICAN FOSSIL BRACHIOPODA. [bull. 87. 



the development of its brachidiiini. This is partiiilly resorbed and 

 ehan^yed in form, and to it is tlien added hiterally the two spirals and 

 medially the simple or, in the higher forms, the complex processes, or 

 jugnm. The vohitions of the spirals in the oldest genera geologically 

 are very few, but subsecjuentl}^ they become more numerous, and attain 

 their maximum in the long-hinged Devonian and Carboniferous spirifers, 

 where 35 volutions have been observed, with 24 in Atrypa. 



The form of tlie paired spirals varies but little except under the ueeeasity of con- 

 formiu":; to the interior cavity of the valves. Their iuclination and direction is a 

 feature of much vsiguificance ^vhen considered with reference to tlie development of 

 the entire shell. It is the loop, or to employ a term more appropriate in view of the 

 homologies of the spire-bearing and loop-bearing shells, the jugnm, however, which 

 is subject to the most frequent variations in form, and Avhich serves as the generic 

 index. When the spirals are directed outward toward the lateral margins of the 

 valves, the jugum seems to be much more variable than in shells where the spirals 

 are introverted or take some intermediate position. In the latter there is a much 

 greater variation in the position of the loop upon the primary lamelhe than occurs 

 in the former. ' 



GENERAL DEVELOPMENT. 



In the preceding images it is shown that the four types of pedicle 

 openings which serve as the i^rime characters in distinguishing the 

 four orders, Atremata, !Neotremata, Protremata, and Telotremata, are 

 present in the oldest division of the Cambrian, the Olenellus zone. 

 From the pre-Cambriau sedimentary rocks, or Algonkian system, prac- 

 tically no fossils are known, though there is evidence in them that 

 life existed. The fact that the Olenellus zone has a varied marine 

 fauna alone indicates that the sea during Algonkian times must have 

 swarmed with living things. When the enormous time represented by 

 the great thickness of Xorth American pre-Cambrian sediments is 

 considered, or that of Bohemia, it is evident that ample time elapsed 

 ior life to attain the degree of complexity manifested in the basal Cam- 

 brian zone. Kayser says that this i^re-Cambriau time was '' probably 

 so long that the beginning of the Cambrian period may be considered 

 as comparatively a recent event."- Van Hise, in writing on the same 

 subject, says:^ 



If geological history were to be divided into three approximately equal divisions, 

 these divisions would not imjjrobably be the time of the Archean, the time of the 

 clastic series between the Archean and the Cambrian, and the time of Cambrian and 

 post-Cambrian. In this connection it is well to recall that many years ago Log;ni 

 suggested that the thickness of the Laureutian and Huronian may surpass that of 

 all succeeding formations, and that the appearance of the so-called Primordial 

 fauna may be considered as a comparatively modern event. 



In the Lower Cambrian there are not many species of brachiopods, 

 nor is the specific differentiation in any order very varied, indicating 



I Hall auil Clarke, Pala'ontology of Kew York, Vol. Vni, Part II, 1895, p. 343. 



'' Text-Book of Comiiarative Geology, 3893, p. 13. 



=* Sixteenth Anu. Eept. TT. S. Geol. Survey, Part I, liS'JO, p. 7G0. 



