752 SUMMARY OF CURRENT RESEARCHES RELATING TO 



earlier forms, thougli dormant, and, as it were, lying in abeyance, and 

 to be detected only by a close scrutiny." 



The general form of Pourtalesia is unlike that of any other known 

 Echinid ; it has the form of an inverted short-necked bottle ; from the 

 side the anterior line is seen to be bluntly truncated, while the dorsal 

 surface is marked posteriorly by a deep depression, behind which 

 there is a truncated caudal prolongation. Anteriorly the test is 

 suddenly bent inwards and backwards, so as to form a deep ovoidal 

 recess leading to the mouth. It would seem as if " this anomalous 

 configuration " were due to " the dorsal portion of the body having 

 moved forwards beyond the normal measure, and so as to leave behind 

 the subanal part of the ventral portion, and as though its forepart 

 produced into a rostrum projecting ventrally and compressed from 

 both sides, had been drawn, by invagination, into the peritoneal 

 cavity." 



The perisomatic portion is next dealt with, and here perhaps we 

 have the most anomalous condition of parts, for it is found that two of 

 the interradii unite in the middle line and so form a continuous 

 broad ring passing round the middle of the body ; this arrangement 

 appears also to be found in Spatagocystis, but it is only seen in 

 P.jeffreysi and P. laguncula among the species of the genus Pourtalesia 

 as defined at present. Loven makes the interesting remark that 

 " once before, early in Mesozoic time, for a while and not unlike a 

 trial soon given up, a structure resembling this w^as seen in the 

 CoUyritidse, but imperfect, the ring being open ventrally and closed 

 dorsally only." As it obtains in P.jeffreysi, the author thinks that 

 the radiate disposition of the skeletal elements is destroyed in an 

 essential degree, " and a tendency betrays itself towards an annular 

 differentiation of the bilaterally symmetrical constituents of the 

 cylindroid skeleton." 



The peristome likewise presents us with some very extraordinary 

 characters ; the structure of which leads us to think " that what is 

 going on here may be looked upon as the first move, so to speak, 

 towards forming a rudimental mouth, a cavum oris, the invaginated 

 parts of which, if they were flexible and provided with muscles, might 

 be protruded like a proboscis." Without here going into details we 

 can only say that the author establishes his proposition that the joint 

 participation of the ambulacra in the formation of the peristome, and 

 the uninterrupted sequence of their plates does not obtain in the genus 

 under consideration. 



The characteristic sensory organs (sphasridia) first detected by 

 Loven in the region of the mouth of Echinids, are not, as in most, 

 arranged in Pourtalesia in all the five ambulacra, but are absent from 

 No. Ill,, or that which is anterior and odd. In Pourtalesia, however, 

 the ambulacrum in question is raised above the level of the peristome, 

 so that we see that " of whatever nature the special changes in the 

 surrounding water may be that their ciliated epithelium has to watch 

 for, these changes seem to be of essential moment to the animal, solely 

 when they take place in the vicinity of the mouth, or between the 

 under surface and the ground on which the animal has to find its food." 



