254 Introduction to Animal Morphology. 



of a pair of spines, one on each side of the ventral 

 mantle lobe. 



Of the 2000 species known, only 80 are living ; the others 

 are mostly palaeozoic. They are more abundant in the 

 Southern than in the Northern Hemisphere, and live at 

 depths of 10-200 fathoms. The horny shelled forms are 

 littoral and sub-littoral. They are most abundantly found 

 on a stony bottom, and feed on Diatoms, Sponges, &c.) This 

 division includes two orders : — 



I . Ecardines {van der Hoeveii) — intestine long, with a lateral 

 anus ; shell hingeless ; no arm supports ; a vasiform heart ; 

 mantle margin ununited ; generative glands in the perivisceral 

 cavity. Herein are the families: — i. Lingulidse — horny, 

 tubular shells, with a muscular peduncle, springing between the 

 beaks of the two nearly equal shell valves, ex. Lingula, from 

 the Pacific. 2. Discinidae — chitinous shell, with an opening 

 in the under valve (Discina, Pacific). 3. Craniidae — calca- 

 reous shell adherent by the under valve (Crania, Atlantic). 



Order 2. Testicardines {Gegcjibaur) — digestive canal short, 

 aproctous ; mantle lobes united behind ; shell calcareous, 

 with a hinge line and prismatic structure. This includes the 

 following families : — Rhynchonellidoe — arm loop calcareous, 

 of two parallel crura ; shell with no area ; hinge line curved 

 or straight ; both valves convex and radially ribbed ; rarely 

 punctated ; the beak in Rhynchonella is imperforate (Medi- 

 terranean and Pacific). 2. Terebratulidae — arm support strong, 

 looped, never spirally developed ; shell biconvex, perforate 

 at the beak, and punctated ; of the included genera the shell 

 is directly adherent to stones in Thecidium, attached by a 

 peduncle in others ; the loop is very short in Terebratula, 

 longer and reflected in Waldheimia, attached to the septum 

 dorsale in the long, narrow Bouchardia ; the loop for the 

 sigmoid arms in the minute Morrisia is not reflected. Kraussia 

 has small oral arms and a transversely ovate shell, and some- 

 times a branched apophysis. Megerlia has the loop trebly 

 attached to the hinge plate, and by two slips to the septum. 



Of extinct families Spiriferidae (Palaeozoic) has spirally 

 coiled diverging arm supports and an impunctate shell. 



