272 SYNOPTIC COLLECTION. 



very early this notch is surrounded by shell layers, 

 although no specimen showing this holoperipheral growth 

 has been found. This is probably due to the fact ' that 

 when very young the animal becomes attached to a rock 

 (No. 663) by its pedicle valve which causes the pedicle 

 to disappear and with it the notch and the circular layers 

 if any such exist. 



BRACHIOPODA. PROTREMATA. 



The Protremata may have arisen from some Paterina- 

 like ancestor which lived in pre-Cambrian times, or may 

 have branched off from the Neotremata at a later period. 

 The forms known to us illustrate specialization of struc- 

 ture brought about through acceleration in development. 



One of the more generalized forms is Kutorgina cingu- 

 lata Billings (PL 664, figs. 1-3). Here the hinge line is 

 straight (fig. i), and there is a narrow rudimentary cardinal 

 area on the pedicle valve (figs, i, 2,/z'). This valve (fig. 

 3) rises above the brachial (figs. 1,2, bv) and a large open- 

 ing is left for the passage of the pedicle (fig. 2) which is 

 only slightly restricted by the deltidium. This plate in 

 the Protremata is formed in the embryo and is a shell 

 growth from the brachial side of the body which later 

 becomes attached to the pedicle valve. 1 The teeth in 

 Kutorgina are primitive and are situated at the outer ends 

 of the cardinal area. Figures illustrating the stages of 

 development are given under Thecidium (see p. 273). 



The brachial valve in the Protremata, as in the other 

 orders, is less differentiated than the pedicle valve, and 

 shows the protegulum as in the Atremata. When ex- 

 tremely young, the shell probably passes through a long- 

 hinged or Paterina stage, but in the early nepionic stage 

 of most genera the pedicle is surrounded by shell layers 



1 See Beecher, Amer. Journ. Sci., (3), XLIV, 1892, pp. 142-147. 



