BRACHIOPODA. 
325 
included under the old fiimily designation introduced by Kutorga in 1848, 
SiPHONOTRETIDJE 
Crania and its allies (Craniell\, 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 Crania 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 and Craniella Hamiltotiice, fi-oin 1.5 to .5 min. in diameter, 
ai-e fully cemented. Examples of Pholidops Hamiltoniw, not above .5 mm. in diameter, give no indication 
of a jiedicle-jiassage or surface characters not present in the adult. 
t Some species of Pholidops {P. arenaria, P. linguloides) have a terminal submarginal apex; and their 
resemblance exteiiorly to the oboloiils is very striking. This is, however, no more than a resemblance, as 
they show, on the under side, the same mode of peripheral growth beneath the beak as the othei- forms of 
the genus in which the umbones are more nearly central. 
