118 PANAMIC DEEP SEA ECHINI. 



primordials and form part of the ridge surrounding the actinal system 

 (PI. 65, fig. 6), a rudimentary auricle perhaps, which is greatly developed 

 in some fossil species of the group, and has led to the supposition that they 

 might he provided with teeth. 



This crowding results in part from the exclusion of the posterior lateral 

 primordial plates from the actinostome hy the adjoining posterior lateral 

 ambulacral plates in Conolampas (PI. 65, fig. 6), thus forming a striking con- 

 trast to the arrangement of the primordial plates in Echinolampas (PI. 65, 

 fig. ^). In Echinolampas the bourrelets are less crowded with tubercles. 



The extent of the crowding of the actinal plates by the coronal plates 

 is well seen from the great height of the actinal ambulacral plates and 

 the disappearance of the pairs of pores of the older plates and the great 

 irregularity' of the ambulacral plates (PI. 65, fig. 6'), which have been twisted 

 and pushed out of place and broken into numerous intermediate plates by 

 this great thrust. The irregularity of the phyllodes thus formed in Cono- 

 lampas is very apparent when seen from the exterior ; the pores are placed 

 in all possible positions (PI. 65, fig. 5), only forming most irregular pairs of 

 lanceolate lines which extend towards the ambitus until the ambulacra 

 again assume their normal structure. 



This irregularity is less pronounced in Echinolampas, as can be seen 

 (PI. 65, figs, i, ;?) from the more distant position of the pores and the 

 comparative regularity in the ambulacral plates even at the phyllodes. The 

 greater crowding of the coronal plates in Conolampas (PI. 65, fig. G) is 

 also accompanied by the greater development of the bourrelets and the 

 dense packing of the secondary tubercles on the actinal face of the primor- 

 dial plates ; in Neolampas (PI. 64, fig. 6) the bourrelets are practically in an 

 embryonic stage, being only slightly accentuated by the packing of the 

 secondary tubercles on the primordial plates and by the formation of very 

 rudimentary phyllodes which do not extend further than to the fourth am- 

 bulacral plate, while in Echinolampas, in a specimen of 50 mm. they extend 

 to the sixteenth or seventeenth plate, and in a Conolampas of 90 mm. they 

 extend as far as the fortieth ambulacral plate. In Rhj/nchopi/gus pacijicus tlie 

 oldest actinal ambulacral plates are quite large and have onlj- one pair of 

 pores. 



In a young specimen of Echinolampas of 4 mm. (PI. 64, figs. ^, 3) the 

 actinal ambulacral plates have onl^- two pairs of pores, and are formed 

 by the ankylosis of the two oldest of the ambulacral plates ; a trace of the 



