80 SYNOPSIS OF AMERICAN FOSSIL HRACIIIOPODA. I bull. 87. 



Greatest represeiitatiou in both genera and species was during the 

 Ordovician, after which generic diftercutiation was practically restricted 

 to the DisciuidiC and Craniidic. Crania persisted tliroughont the post- 

 Ordovician, and for longevity equals the atrematous genus Lingula. 



The percentage of widely dispersed species is about the same as in 

 the Atremata, and likewise is greatest in those families with the longest 

 phylogenetic history, as Acrotretidic, Discinida?, and Craniidie. 



Development was along two lines. In one a broad fissure (the most 

 primitive condition of the pedicle opening in this order) is retained as 

 a mature character (Trematida^,). Later geologically, and at the matu- 

 rity of the individual in derived forms, the fissure is gradually closed 

 posteriorly, leaving a long, narrow slit, at one end of which the pedicle 

 emerges (Discinida^). The other line ( Acrotretacea) probably developed 

 and inherited holoperipheral growth in the ventral valve, very rapidly 

 producing a small subcentral circular foramen, since this feature is 

 already well developed in the Lower Cambrian Acrotretidu^, and in 

 advance of the greatest development of the Discinid*. It is probably 

 this second branch that gave origin to the degraded family Craniidie. 

 The protegulum in the dorsal valve of Acrotretacea is probably alwaj'S 

 marginal, whereas in the Discinacea it is always more or less central. 



It is remarkable that Crania, so unlike other living brachiopods 

 and occurring abundantly in the seas of to-day, has never been com- 

 pletely studied developmen tally or ontogenetically. The taxonomic 

 position of the Craniid.e is therefore not actually determined, and Hall 

 and Clarke incline to follow Waagen in regarding the Craniacea as 

 equivalent in rank to the Atremata and Neotremata. These authors 

 write : ' 



It is nevertheless to be observed that no trace of a former pedicle-slit incision or 

 perforation is found on mature or immature shells, and it would be difficult to com- 

 prehend in what manner such an essential modification of the shell could be wholly 

 concealed by later growth. Were the pedicle marginal in primitive growth stages, 

 and subsequently atrophied, the obliteration of the marginal opening by later 

 resorption and growth would be a readily intelligible process. There is, hence, in 

 this default of evidence, a good reason to donbt the close affinities of Crania and 

 Pholidops to the Diacaulia [= Neotremata]. Present knowledge would seem to 

 indicate that they were primarily of the type of the Mesocaulia [= Atremata], and 

 that their resemblance to the Diacaulia is wholly of secondary growth. Waagen's 

 term for this group, Gastropegmata (or Craniacea), may therefore prove to be 

 equivalent to each of these other two divisions. 



Brachiopod embryology demands a ])edicle in the early stages of 

 Crania. The ventral valve carries the pedicle, and it is always this 

 valve which is attached by cementation or otherwise. The writer has 

 observed in Yale University Museum a specimen of Pholidops ovata with 

 a cicatrix of attachment, around which point growth is holoperipheral, 

 as in all Neotremata. Specimens of Pholidops are sometimes pre- 

 served with both valves in position and delicately attached to Bryozoa^ 



' Palaeontology of New York, Vol. VIII, Part n, 1895, p. 325. 



