Report of the State Geologist, 



183 



the apex nearly to fche basal margin of the delthyrium, but also 

 builds up on the inner side of this plate a tubular sheath, open 

 along its ventral surface and which probably inclosed the atro- 

 phied pedicle <>f the animal. This peculiar tubular structure may 

 be termed the syrinx. 



Fig. 66. 



Interior of pedicle-valve of Syringothyris. 

 split tube or syrinx. 



t, the 



Fig &7.— Syringothyri8 typa. The 

 interior of the umbonal portion 

 of the pedicle-valve, showing the 

 split-tube or syrinx attached to 

 the transverse plate filling the 

 delthyrium . 



The covering of the delthyrium in the brachial valve (chilidium) 

 which is always a single plate is of later growth than the delti- 

 dium, and is therefore not a primitive structure. 



Fig. 68.— Cardinal view of a portion of the valves of Orthotheies subplana ; showing a 

 remarkable development of the chilidium. 



THE INTERNAL CONFIGURATION OF THE VALVES. 



Articulating Apophyses. 



Articulation of the valves is mainly effected by means of teeth 

 and sockets, the former on the pedicle-valve, the latter on the 

 brachial valve. In inarticulate genera the valves are usually held 

 in apposition simply by muscular contraction, and seldom is any 

 tendency to the formation of articulating processes manifested. 

 Occasionally low bosses are formed on the cardinal margin of the 

 pedicle-valve (Trimerella, Barroisella, Tomasina ; Plate 1, fig. 

 14), which probably subserved to a certain extent the pur- 



35 



