108 



SYNOPSIS OF AMERICAN FOSSIL BRACHIOPODA. 



[bull. 87. 



\^ 



and Discinisca, and, like the former stages, is undoubtedly (;ommon to 

 all bracliiopods, exci'i)t, i>erli<ips, Tphidea. 



Gwyiiia is an adult living representative of this stage, and never 

 develops any higher type of brachial structure. Dyscolia also belongs 

 here, since it has a discoid lophopliore surrounded by a marginal fringe 

 of tentacles (Fischer an<l (T'dilert"). It is possibly a little more 

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

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



Tiie absence of septum, hinge-i)latc, and dental plates are other 

 primitive characters belonging to Dyscolia. 



SCHIZOLOPHUS STAGE. 



After the completion of the trocholophus stage in all brachiojiods, 

 escei)t 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 tirst 

 accomplished by an intro- 

 version of the anterior 

 median edge, thus divid- 

 ing the lophoiihore into 

 two lobes, and suggesting 

 the name scMzolophtis for 

 this type. (See figs, i'c, g, 



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- 

 bratellidii',wlii('li has been 

 c ailed the (;i s tell i for m 

 stage (fig. 3c). Terebratulina (fig. 3//), Glottidia (fig. 56'), and other 

 higher forms, also have correspoiuling schizoloi)hian stages, but are 

 without the median septum. Lacazclla medlterruncii presents a similar 

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

 sil genera Davidsonella and Thecidella of the Thecidiida', and Zellania 

 of the Terebiatcllidii', lu^ver devel(>i)ed beyond the schizolophns stage, 

 and they must therefore be considered as quite j)rimitive genera in their 

 respective families. 



From this point the further development and complication of arm 

 structure proceeds in three distinct diverging lines, producing the 



\^^ Taxolophus. 



Trocholophus. 



Scliiz(»loi>hus 



Ptycholophus. 



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

 Cistella, and Megathyris. a,b,c, d, stages intliegrowthof the 

 lophopliore in Thecidea (Lacazella) mediterranea , eul. {a-c after 

 Kovalevski, d, after LacazeDuthicra). e, /, early stages of 

 lophophoreof Cistella neapolitana, eTi\. (alter Kovalevski). g, 

 adult lophojihore of Cistella (C eittellida), aul. (after David- 

 son), h, lahial appendages of Megathyris decollata.vnl. (after 

 Davidson). 



