THE BRACHIOPODA 43 



simple bar, as in the two other families : its least complex 

 form is a Y, but it may be scissors-shaped or have still 

 more elaborate forms. Genera in this family usually 

 have a rounded form and short hinge-line. Meristina is a 

 Silurian genus, with a median dorsal septum, and paired 

 ventral septa (the converse of the contemporary pen- 

 tamerids); Athyris and Semimda are Carboniferous forms* 

 the latter mimicking the contemporary Dielasma, but dis- 

 tinguishable from it not only by its internal spirals, but 

 also by its non-punctate shell. 



The Spiriferacea appear in the Ordovician, where only 

 primitive atrypids are found ; all families are represented 

 from the Silurian to the Carboniferous, when the atrypids 

 die out. The other two families survive to the earlier 

 part of the Jurassic period, some -species of Spiriferina 

 being quite common in certain zones of the Lias. The 

 Whitbian age (time of lower part of Upper Lias) seems 

 the date of the final extinction of all Palaeozoic types 

 of brachiopods not only spiriferids (Spiriferina) and 

 athyrids (Koninckella), but also leptaenids (Cadomella) 

 making their last appearance. Certain minute forms in 

 the Inferior Oolite that have been ascribed to the 

 spiriferids really belong to other groups (Megathyrinae 

 and Rhynchonellids). 



Short Bibliography. 



This and succeeding short bibliographies are intended 

 to give the student some idea of the original works from 

 which he may obtain more detailed information than can 

 be given in any textbook, and of which he must study 

 those which are relevant if he is engaged in research 

 even the simple research involved in naming accurately 

 the fossils he collects. In selecting works for these 

 lists, regard has been had to accessibility (to English 



