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



the development of its bracbidium. This is partially resorbed and 

 changed in form, and to it is then added laterally the two spirals and 

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

 jngnm. The volutions of the spirals in the oldest genera geologically 

 are very few, but subsequently 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 the paired spirals varies but little except under the necessity of con- 

 forming to the interior cavity of the valves. Their inclination and direction is a 

 feature of much significance when considered with reference to the 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 jiigum, however, which 

 is subject to the most frequent variations in form, and which 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. ' 



I GENERAL DEVELOPMENT. 



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

 openings which serve as the prime 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-Carabrian 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 J^orth American pre-Cambrian sediments is 

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

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

 brian zone. Kayser says that this pre-Cambrian time was "probably 

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

 as comparatively a recent event." ^ Van Hise, in writing on th^ same 

 subject, says:^ 



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

 these divisions would not improbably 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-<Jambrian. In this connection it is well to recall that many years ago Logan 

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

 all succeeding formations, and that the appearance of the so-calletl 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 dili'erentiation in any order very varied, indicating 



' Hall and Clarke, Palaeontology of New York, Vol. Vni, Part II, 1895, p. 343. 



"Text-Book of Comparative Geology, 1803, p. 13. 



'^ Sixteeuth Ann. Kept. U. S. Geol. Survey, Part 1, 1890, p. 700. 



