FAMILIES OF LOOP-BEARING BRACHIOPODA 291 



of two lamellffi, or descending branches, from the points of 

 tlie crura, uniting in the median line. The central portion 

 may be narrow or medially expanded. In some genera, re- 

 curved ascending branches are produced by the partial resorp- 

 tion of the broad band or plate forming the connection 

 between the descending branches. The cirri in early stages 

 of the animal are centrifugal or directed outward. The 

 growth of the loop in Terebratulina has been illustrated by 

 Morse. -"^ 



Terehratnla (^Liothyrina) and Terebratulina may be selected 

 as best representing the Terebratulidce ; for Byscolia, Agul- 

 hasia, and Eucalathis do not represent the highest develop- 

 ment of the family type, but must be regarded as degraded 

 forms. Among fossil genera Cryptonella, Megalanteris, Bie- 

 lasma, Centronella, Re^isselceria, Stri7igoce2?halus, and some 

 others, probably belong here. The following sub-families can 

 be recognized : (1) the Centronellinse, (2) Stringocephalinfe, 

 (3) Terebratulinae, and (4) Dj-scoliinae. The adult arm 

 structure in Byscolia is homologous with early larval features 

 in Terebratulina; also the cirri are centrifugal or directed 

 outward, as in early stages of Terebratulina, and not cen- 

 tripetal as in larval Mayellania. 



The Terebratellidce. 



The loop in the second family, for which the name Tere- 

 bratellidse is retained, undergoes a series of metamorphoses 

 while attached to a dorsal septum during the larval and im- 

 mature stages of the animal, and in the higher forms results 

 in a loop of secondary growth much like the primary loop 

 of some of the early genera of the Terebratulidce. The cirri 

 in larval stages of the animal are centripetal or directed 

 inwardly. 



In one division of the Terebratellidae the stages of growth 

 may be correlated with the adult loops in the genera Gwynia, 

 Cistella, Platidia, Ismenia, Muhlfeldtia, Tercbratalia,* and 

 Dallina;-f while in another division a quite different series 



* Type Terehratula transversa G. B. Sowerby. 

 ■f Type Terehratula septigera Lov^n. 



