110 



SYNOPSIS OF AMERICAN FOSSIL URACHIOPODA. 



llULL. 87. 



a, i> c 



Fig. 4. — Metamorphoses oC tlui brachidiiiiu iu Diclaiiaa lurijida,t>uL 

 (after Beecher ami Schuchert). 



and space allowed for the median arm — lig. 4ia^d. In a 8[)ire-beariug 

 genus like Zygospira this is more obvious, for here the transverse pro- 

 cess or jugum is clear- 

 ly the result of the 

 growth and resorp- 

 tion of the centronel- 

 liform loop to admit 

 the spiral! a. 



The calcareous loop in Terebratuliua and Liotliyrina is only a posterior 

 basal supjjort, and does not repeat the outline of the cirrated margin of 

 the lophophore, exclusive of the arm. Therefore it is impossible 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 Terebratuliua 5 

 but the lophophores are quite distinct iu 

 each, the former being of the trocholophus 

 type and the latter belonging to the plec- 

 tolophus. 



SPIROLOPHUS STAGE. 



Taxolophus. 



The last type to be noticed is the one iu 

 which there are two separate coiled arms, 

 each with a row of cirri on one edge only — 

 fig. 5d, e. It embraces the greater part of 

 the families of brachiopods in the orders 

 Telotremata and Protremata, and includes 

 all the living species in the orders Atre- 

 mata and Keotremata. 



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 taxolophus, trocholophus, 

 and schizolophus stages may be deter- 

 mined — fig. 5a, b, c. The separation and 

 growth of the spiral arms seem to be due to 

 the widening or expansion of the median 



Trocholophus. 



Schizolophus. 



Spirolophus. 



Fig. 5. — Early stages of lophophore of 



and Hemithjris. a, b, c, early stages of 

 lophophore of Glottidia audebarti, oiil. 

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

 gula (after Woodward) . e, adult brachia 

 iu Hemithyris psittacea (after Hancock). 



lobe or tentacle, on each side of which is Glottidia and adult bracWa in Lingula 



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 iu understanding the development and com- 

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

 The earliest stage observed (tig. Ga) has the form of a simple terebratu- 

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



