BRACHIOPODA 



183 



Sub-genus Dictyothyris (Jurassic) with fine radial ribs and con- 

 centric lines; ex. D. coarctata. Dielasma (Devonian to Permian) and 

 Ccenothyris (Trias) are distinguished from the Terebratulce of Jurassic 

 and later formations mainly by the possession of well-developed dental 

 plates. Ex. Dielasma hastatum, Carboniferous ; Coenothyris vulgaris, 

 Trias. 



Terebratulina (fig. 87). Form similar to Terebratida. Orna- 

 mented with fine radiating ribs. 

 Umbo short, foramen large, delti- 

 dium small. Two ear-like processes 

 at the sides of the dorsal umbo. 

 Brachial loop short, with a ring 

 formed by a band between the two 

 branches. Jurassic to present day. 

 Ex. T. striata, Chalk. 



Magellania ( = Waldheimia) 



(fig. 72). Distinguished from Tere- 

 bratula by the longer brachial loop, 

 extending to at least half the 

 length of the shell, and by a median 

 septum in the dorsal valve, which is, however, sometimes rudimen- 

 tary. The shell may be smooth, or with folds, or radially ribbed. 

 Lias to present day. Ex. M. flavescens, Recent; M. cornuta, Lias; 

 M. impressa, Oxford Clay. 



Magellania, as defined above, includes a large number of species 

 which have been divided into several groups ; by some authors 

 these divisions are regarded as genera or sub-genera, some of the 

 more important being : — Eudesia (ex. E. cardium, Great Oolite) ; 

 Zeilleria (ex. Z. cornuta, Lias); Ornithella ( = Microthyris) (ex. 0. or- 

 nithocephala, Cornbrash) ; Aulacothyris (ex. A. resitpinata, Lias). 



Fig. 87. Terebratulina caput- 

 serpentis. Interior of dorsal 

 valve. Eecent. (From Wood- 

 ward. ) x 2. 



Terebratella. Shell oval, usually with radiating ribs. Ven- 

 tral valve very convex ; dorsal more or less flattened. Hinge-line 

 straight or slightly curved ; an area present. Umbo with a large 

 foramen, and deltidium below. Brachial skeleton similar to Magel- 

 lania, but descending branches joined by a band to a septum in the 

 middle of the dorsal valve. Lias to present day. Ex. T. pectita, 

 Upper Greensand. 



