Report of the State Geologist. 



537 



The Development of the Shell. 

 There are at least two types of shell development among the 

 articulate brachiopods. Oneof these is exemplified by Theoidium, 

 which has been shown by Beeoheb to be a living representative of 

 the extinct palaeozoic group provided with a deltidium formed 

 of a si ogle plate or sheath ; likeSTBOPHOMENA and its allies. (Plate 



Fig. 204. 



Fig. 206. 



Fig. 209. 



Fig. 205. 



Fig. 207. 



Fig. 



Fig. 210. 



Thecidium (La.cazella) mediterraneum. 



Fig. 204.— Cephalula, dorsal side; showing below, the cephalic segment with eye-spots, and on 



the upper segment the dorsal shell-plate. 

 Fig. 205.— Dorso- ventral longitudinal section of cephalula; below is the cephalic segment, at the 



right the dorsal mantle lobe, the thickened line on its inner margin representing the beginning 



of the dorsal valve, and the similar line on the adjoining side of the body the incipient 



deltidium. 

 Fig. 206.— A later growth stage (typembryo), in which the mantle lobes have turned downward. 



The body shell plate is seen in theupper part of the figure. 

 Fig. 207. — Dorso-ventral longitudinal section of the preceding; showing the inversion of both 



mantle lobes. The relations of the dorsal and body (deltidium) plates are shown by the heavy 



lines at the right. The ventral plate is also seen on the lobe at the left. 

 Fig. 208.— Profile of a very young specimen of Leptcena rhomboidalis oriented to correspond 



with the foregoing figures. 

 Figs. 209, 210.— Views of ad"lt Thecidium CLacazella) mediterraneum similarly placed. 



(Bkecher; figs. 204-207, adapted from Kovalevski.) 



1 6, Figs. 1-18.) The other is represented by Cistella, which typifies 

 the group bearing discrete deltidial plates. In both of these, the 

 first traces of the shell are found in the neoembryo. In the 

 cephalula stage of Thecidium {Lacazella) mediterraneum the dor- 

 sal lobe of the mantle develops more rapidly than the ventral, 

 and its inner surface bears a flat shell-plate. A similar plate is 

 1891. 68 89 



