THE BRACHIOPODA 31 



tions near the umbones. Here and there, especially near 

 the sides, may be seen round spots, proved by comparison 

 with better-preserved specimens to be the stumps of 

 long curved spines. These spines are the special character- 

 istics of the family Productidtz, and they served perhaps to 

 anchor these brachiopods to the stems of crinoids or other 

 suitable objects, or perhaps to support them like springs 

 on the sea-bottom, thus enabling them to dispense with a 

 pedicle. 



Internally, Productus does not differ greatly from 

 Leptana : the divaricator-impressions in the ventral valve 

 are more widely separated, and in front of each one there 

 is a spiral thickening of the interior of the valve like that 

 of Davidsonia ; in the dorsal valve there is a pair of some- 

 what similar, reniform (kidney-shaped) elevations. 



In the genus Chonetes (Ord.-Perm.) we seem to 

 see a link with the Leptcenida. It has similar spines to 

 Productus on the cardinal margin of its ventral valve only, 

 and it retains cardinal areas and delthyrium. On the 

 other hand, Etheridgina (known only in the Carboniferous 

 of Scotland) shows a further modification of the Productus 

 method of fixation : the spines grip the crinoid stem 

 tightly, and the ventral valve becomes cemented to it. 



FIG. 7. VARIOUS BRACHIOPODS (continued} 



bonal region of Orthotetes, ground down to show dental plates and 

 septum (black), i, Internal cast of ventral valve of Meekella, dental 

 plates black, j, Conchidium knighti (]. Sowtrby), Silurian, dorsal 

 view. /, Naturally split section ; /', cross-section, (xj.) k, Spirifer 

 striatus (Martin), Lower Carboniferous, dorsal view, with test paitly 

 removed, showing spiralia. (xj.) /, Cyrtia exporrecta (Wahlen- 

 berg), Silurian, dorsal view, (xj.) m, m', Atrypa reticularis 

 (Linne"), Silurian, side view and cross-section showing spiralia. (x.) 

 A, Cardinal area; C, crus ; C.P., cardinal process; del., deltidium ; 

 D.V., dorsal valve; H.P., crural plates (cruralium) ; ./?, reniform 

 impression ; S, septum ; SP, spondylium (dental plates) ; T, hinge- 

 tooth ; V.V., ventral valve; X, septal plate. <z, b, After Walcott ; 

 f-i, after Thomas ; the rest after Davidson. 



