BRACHIOPODA. 325 



included under the old family designation introduced by Kutorga in 1848, 



SipBONOTRETIDjS 



Crania and its allies (Craniella, Pseudocrania, Pholidops) constitute a group 

 in which there is, thus far, no satisfactory evidence of the existence of the 

 pedicle, and we are left to the inference that this organ became atrophied at 

 a very early growth-stage. The study of recent Cranias has not yet deter- 

 mined this point, but this will probably be ultimately accomplished. At 

 whatever stage of growth the pedicle was lost, we may infer that its disap- 

 pearance, in Crania, and generally in Craniella, was directly followed by a solid 

 fixation of the animal by the substance of one of the valves. In Pholidops 

 there was no such cementation, but at a correspondingly early stage the shell 

 became wholly independent. All these shells with central or subcentral beaks 

 have an external resemblance to Orbiculoidea ; the formation of the secondary 

 growth of the valves behind the apices or position of the protoconch, is a fur- 

 ther substantial agreement with the Diacaulia as contrasted with the abbrevi- 

 ated posterior peripheral shell-growth in the Mesocaulia (Lingula, Obolus). 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 comprehend 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 doubt the close 

 affinities of Cranla and Pholidops to the Diacaulia. Present knowledge would 

 seem to indicate that they were primarily of the type of the Mesocaulia, and 

 that their resemblance to the Diacaulia is wholly of secondary growth.f 



* Quite early conditions of Crania siluriana aixl Craniella ffamiltoHiw, from 1.5 to .5 mm. in diameter, 

 are fully cemented. Examjilea of Pholidops HamiltimuB, not above .5 mm. in diameter, give no indication 

 of a [)edicle-])a8sage or sui-face characters not present in the adult. 



t Some species of Pholidops (P. arenaria, P. livgiiloidex) have a terminal submarginal apex ; and their 

 resemblacice exteriorly to the oboloids is very striking. This is, however, no more than a i-eserablance, as 

 they show, on the under side, the same mode of peripheral growth beneath the beak as the other forms of 

 the genna in which the umbones are more nearly central. 



