THE BRACHIOPODA 25 



umbo and at a distance from the margin. In the more 

 typical sub-orders the shell is horny and phosphatic. 



SUB-ORDER i. Acrotretacea. Shells but little 

 changed from obolids in shape, especially the dorsal 

 valve. Older Palaeozoic. Chief genera : Acrotreta, in 

 which the ventral valve has a high cardinal area (Camb.- 

 Ord.),* and Siphonotreta in which the pedicle-opening 

 leads into an internal tube, and the shell is perforated by 

 pores which extend into hollow spines (Ord.-Sil.). 



SUB-ORDER 2. Discinacea. With each valve more or 

 less conical in shape, umbo eccentric. In Trematis of the 

 Ordovician the pedicle-aperture is a long slit extending 

 from the posterior margin to the umbo ; but in Orbicu- 

 loidea (Ord.-Cret.) it becomes closed-in as growth pro- 

 ceeds by a plate called the listrium, and the short pedicle 

 passes obliquely through the slit that remains. In exist- 

 ing seas this sub-order is represented by Discinisca and 

 Discina. 



SUB-ORDER 3. Craniacea. Aberrant forms, in which 

 the shell is calcareous and the pedicle and pedicle- 

 opening are lost, fixation by cementation taking the 

 place of fixation by pedicle. Ordovician to Recent. The 

 principal genus Crania has the range of the sub-order. 



ORDER III. : PROTREMATA. 



Articulate Brachiopoda, with delthyrium restricted by 

 a deltidium. Shells calcareous. No brachial skeleton, 

 except crura in some cases. 



SUB-ORDER i. Strophomenacea. In the Cambrian, 

 this is represented by the little Kutorgina, which, but for 

 its more calcareous shell, broader shape and wide delthy- 

 rium, differs little from the contemporary Inarticulata, for 



" These are obvious abbreviations of the names of the geological 

 systems, tabulated in Appendix I. 



