wider area. The way found was to stretch ciliated grooves from 

 the mouth over the surface of the theca, and then to raise them on 

 armlets or brachiola placed at fit intervals. At the same time, 

 the hydrocoel, it may be inferred from its constant connection 

 with such grooves, sent out branches corresponding with the food- 

 grooves ; from these branches were given off the podia, serving 

 for both respiration and the prehension of food. This wandering 

 of the brachiola away from the mouth marks then the develop- 

 ment of two structures previously unknown among Echinoderms : 

 (1) canals radiating from the hydrocoel along the theca ; (2) an 

 intrabrachial, circumoral, or ventral region of the theca, such as 

 in Crinoids is called the tegmen. These two structures, in one 

 form or another, characterise all Echinoderma other than Amphor- 

 idea and a few Rhombiferi. But it seems that they were inde- 

 pendently developed along many lines. What in the Crinoidea 

 is a mere " tegmen " or pot-lid, comes in Glyptosphaera, in the 

 Edrioasteroidea, and in the Echinoidea, to form the greater part of 

 the test ; and with it the perradial ambulacral vessels extend ; 

 whereas the theca of Aristocystidae, which in Crinoidea becomes the 

 specialised and important calyx or dorsal cup, is in the other orders 

 more and more reduced until in some cases no more of it can be re- 

 cognised than the plates of the anal pyramid or their homologues. 

 Another structure characterising this order, viz. diplopores (p. 41), 

 may possibly have assisted respiration by bringing lacunar blood- 

 vessels into closer contact with the sea- water ; this may have been 

 connected with a less development of podia. The functions of 

 diplopores have not as yet been satisfactorily explained by reference 

 to recent Echinoderms. 



FAMILY 1. SPHAERONIDAE. Primitive Diploporita, in which the food- 

 grooves do not extend from the mouth beyond the adoral circlet of plates. 

 Diplopores diffuse. The included genera show the early stages of tegminal 

 and ambulacral development ; in none of them does more than a single 

 cycle of plates intervene between the oral pole and the bases of the 

 brachiola. The five plates forming the cycle are interradially placed, 

 being separated by the grooves proceeding from the mouth to the brachioles. 

 The direction of these grooves is primitively the same as that of the food- 

 grooves in Echinospliaera, viz. one anterior, opposed to the anus, two 

 lateral, each of which soon branches, thus making five grooves in all, 

 with bilateral symmetry. Thus the shape and position of the five inter- 

 vening plates are those characteristic of true orals (see p. 1 24). Genera 

 Sphaeronis, Hisinger (1828 and 1837) ; the name was proposed to replace 

 Echinosphaera of Wahlenberg (1818) for no assigned reason, but was 

 restricted to forms agreeing with Echinus pomum, Gyllenhal, by Joh. 

 Miiller (1854). The species referred to this genus by Angelin (1878) are 

 all Ordovician, and agree in the following characters (Fig. XXXVIII.) : 

 A spheroid or ovoid theca, sessile on a broad base, composed of irregu- 

 lar plates, the mesostereom of which is pierced by regularly formed 



