108 



SYNOPSIS OF AMERICAN FOSSIL BRACHIOPODA. 



[bull. 87. 



\J^ %^ Taxolophus. 



and Discinisca, and, like the former stages, is undoubtedly common to 

 all bia('bio])od8, except, perhaps, Ipbidea. 



Gwj-nia is an adult living rei)resentative of this stage, an<l never 

 develops any higher type of brachial structure. Dyscolia also belongs 

 here, since it has a discoid loi)hophoro surrounded by a marginal fringe 

 of tentacles (Fischer and (Ehlert"). It is possibly a little more 

 advanced than (hvynia, as it has a slight median anterior notch, sug- 

 gesting the beginning of the bilobed structure of the next higher type. 



The absence of septum, hinge-plate, and dental plates are other 

 primitive characters belonging to Dyscolia. 



SCHIZOLOPHUS STAGE. 



After the comi)letion of the trocholophus stage in all brachiopods, 

 except such simple forms as Gwynia and Discolia, no further increase 



in the cirrated edge of 

 the lophophore can occur 

 without some deformation 

 of the circle. This is lirst 

 accomplished by an intro- 

 •versiou of the anterior 

 median edge, thus divid- 

 ing the lophophore into 

 two lobes, and suggesting 

 the name schLoIophus for 

 this type. (See figs. 2c, g, 

 3e,h,5c.) 



Several brachiopods re- 

 tain the schizolophian 

 brachia as an adult char- 

 acter. Of these, Cistella 

 is perhaps the best exam- 

 ple, as it agrees exactly 

 with an early stage of arm 

 structure among the Tere- 

 bratellida^,whichhas been 

 called the cistelliform 

 stage (fig. 3c). Terebratulina (fig. 3/i), Glottidia (fig. 5c), and other 

 higher forms, also have corresponding schizolophian stages, but are 

 without the median sei^tum. Lacazella mediierranea presents a similar 

 larval structure, and in L. harretti it is retained to maturity. The fos- 

 sil genera Davidsonella and Thecidella of the Thecidiidie, and Zellania 

 of the Terebratellidse, never developed beyond the schizolophus stage, 

 and they must therefore be considered as quite primitive genera in their 

 respective families. 



From this point the further development and complication of arm 

 structure proceeds in three distinct diverging lines, producing the 



Trocholophus. 



Scliiz()lo]dius. 



Ptycholophus. 



Fig. 2. — Stages of growth of the lophophore in Thecidea, 

 Cistella, and Megathyris. a, 6, c, d, stages in the growth of the 

 lophophore in Thecidea (Lacazella) mediterranean eul. (a-c after 

 Kovalevski, d, after LacazeDuthiers). c, /, early stages of 

 lophophoreof Cistella neapolitana, eul. (after Kovalevski). <j, 

 adult lophophore of Cistella (0. cittellvla), enl. (after David- 

 son), h, labial appendages of Megathyris decollata, enl. (after 

 Davidson). 



