no 



SYNOPSIS OF AMERICAN FOSSIL BRACHIOPODA. 



I'lG. 4. — Metumorphose.sof tile biachiiliuni iii Didanma ttirgida,en\. 

 (after Beecher aud Schuchert). 



ami space allowed for the median arm — fig. Aa-d. lu a spire-bearing 

 geuus like Zygospira tliis is more obvious, for liere the transverse pro- 

 cess or juguiu is clear- 

 ly the result of the 

 growth aud resorp- 

 tion of the centrouel- 

 liform loop to admit 

 the spiralia. 



The calcareous loop iu Terebratulina and Liothyrina is only a posterior 

 basal support, and does not rei)eat the outline of the cirrated margin of 

 the lophophore, exclusive of the arm. Therefore it is imj)ossible in 

 these and closely allied genera to infer the stage of development of the 

 lophophore from the loop alone. Dyscolia is an excellent example, since 

 the loop is the same as in Terebratulina; 

 but the lophophores are quite distinct in 

 each, the former being of the trocholophus 

 type and the latter belonging to the plec- 

 tolophus. 



SPIROLOPHUS STAGE. 



Schizolophus. 



Taxoiophus. 



Spirolophus. 



Trocholophus. 



The last type to be noticed is the one in 

 which there are two separate coiled arms, 

 each with a row of cirri on one edge only — 

 fig. 5d, e. It embraces the greater i)art of 

 the families of brachiopods in the orders 

 Telotremata and Protremata, and includes 

 all the living species in the orders Atre- 

 mata and Neotremata. 



In the early stages of development of 

 the spiral lophophore there is an agreement 

 with the early stages of the families already 

 noticed, and the taxoiophus, trocholophus, 

 and schizolophus stages may be deter- 

 mined — fig. 5«, hy c. The separation and 

 growth of the spiral arms seem to be due to 

 the widening or expansion of the median 

 lobe or tentacle, on each side of which is 

 the formative tissue for new cirri. This 

 is very apparent in the young Discinisca 

 described by Muller,^^ and the Glottidia 

 described by Brooks.'"' 



The brachidium in Zygospira passes through a series of changes 

 which have been described in detail elsewhere.^ These metamorphoses 

 are of great assistance in understanding the development and com- 

 parative morphology of this feature in other groups of the Spiriferacea. 

 The earliest stage observed (fig. 6a) has the form of a simple terebratu- 

 loid loop, Avhich, from its resemblance to Centronella, was called the 



Fio. 5. — Early stages of lophophore of 

 Glottidia and adult brachia in Liugula 

 and Hemithyris. a, b, c, ciirly stages of 

 lophophore of Glottidia audehatti, onl. 

 (after Brooks), d, adult brachia in Lin- 

 gula (after Woodward), c, adultbraehja 

 in Hemithyris ■psiltacca (after Hancock). 



