516 



Forty-fifth Report on the State Museum. 



The ability of the brachiopods to form calcareous supports for 

 the brachia may be regarded as degenerate in living species. 

 Among extinct species, especially those belonging to the later 

 Pahezoic and earlier Mesozoic eras, are to be found delicate and 

 often exceedingly complicated supports, which were probably 

 continuous with the fleshy arms for the entire extent of the 

 latter. In Spirifer, for example, long spiral cones consisting of 

 many revolutions of an exceedingly tenuous calcareous ribbon 

 are frequently preserved with the utmost delicacy, and 



in 



Fig. 145.-Terebratulina Fig. 146. — Spirifer mucronatus. Interior of 

 caput-serpentis, with brachial valve, showing spirals, 



arms extended. (Bar- 

 rett.) 



Kayseria, Amphiclina and some forms of Athyris from the St. 

 Cassian beds, these spirals are accompanied by an accessory pair, 

 making four in all. The mode of union of these parts, their 

 attachment to the valves and to the various supporting septa, 

 are features of much significance, while the delicacy of such 

 structures renders the determination of these points of great 

 intricacy and difficulty. 



Such calcareous supports, which in any individual are known 

 as the orachiaium, are formed by the interlocking and cementa- 

 tion of the spicules which are disseminatedthrough 

 the canals and cirri of the brachia. The recent 

 brachio pods possess only simple lamellar exten- 

 sions which, in Terebratella, Magellania. Liothy- 

 rina and Terebratulina consist of two lateral 

 processes. These may be united at their anterior 

 extremity (Liothyrina, Terebratilina). or re- 

 of brachial valye of H exec i anteriorlv, the union of the branches 



Liothyrina vitrea. 



- being median or posterior (Magei.lania, Tere 

 bratella. 



[n all these cases the simple apparatus corresponds to only a 

 portion of the first revolution of the ribbon in Spirifer. 

 Rhynchospira. etc. 



68 



Fig. 147. — Interior 



cess; g, brachial 

 supports. (Dav- 

 idson.) 



