4 SVEN LOVEN, ON POURTALESIA, A GENUS OF ECHINOIDBA. 



by the Challenger Expeditioi), iiiid described by Alexandek Agassiz. ') When they 

 arrived, the description of P. Jeffreys! was long finished and the five first plates en- 

 graved. Although the specimens were in a very fragmentary condition, they permitted 

 me to add some facts of importance. 



In the following pages I have laid down the results of my studies on the mate- 

 rials thus afforded, and also some observations, old and new, on certain particulars in 

 the structure of other Echinoidea that have not hitherto met with the attention they 

 deserve, and the knowledge of which is necessary in order fully to appreciate the re- 

 markable characteristics of the Pourtalesia;. 



I. GENERAL FORM OF THE SKELETON. 



The skeleton of tlie Ecliinoidea, its terminology, bilateral symmetry, autero-posterior axis, and peristomal 

 formula. The skeleton of Pourtalesia. 



In the whole of the Echinoidea the skeleton is a hollow sack inclosing the visceral 

 cavity, and constituted by the three distinct systems: the perisomatic or interradial, 

 the ambulacral, and the calycinal or apical, all simultaneously present in the adult 

 animal, and in view outwardly. Each of these systems is composed of a number of 

 more or less flattened ossicles of definite outline, contiguous and arranged in regular 

 order, rarely imbricated, and consisting in most cases of a calcified, reticular and rigid 

 tissue, continually in a state of resorption and renewal, and extending between the 

 dermal tegument with its dependencies, and the subjacent connective tissue with the 

 peripheral nerves, on the outside, and the peritoneal lining of the visceral cavity, on 

 the inside. 



For the sake of brevity I shall here, as in my former work, ^) designate by num- 

 bers the divisions of each of these systems. Thus among the five anabulacra the one 

 which is anterior and frontal will be marked III, the two on its right II and I, and 

 the two on its left IV and V. The interradial areas will be denoted by the numbers 

 1 to 5, counting from the lateral right hand one, 1, to the posterior odd one, 5. Within 

 tlie calycinal system the ossicles contiguous to the ambulacra will bear the correspon- 

 ding numbers of these: I . . . V, while those adjoining the interradia will be numbered 

 accordingly, 1 ... 5. It is of course altogether indifferent how these numbers are applied 

 from the first, provided only the order once chosen be adhered to. It seemed con- 

 venient to make the recognised frontal ambulacrum of the Exocyclic number III, from 

 its having a medial position relatively to each of the two pairs, II and IV, I and V, 

 and from III being the mean of I + V as of II + IV. 



') Proc. Araer. Acad., XIV, 1879, p. 265. — lleport on the Echinoidea of the Voy. of the Challenger, p. 124. 

 -) Etudes sur les Echinoidties. K. Sv. Vetenskaps-Akademiens Handliiigar. Vol. XI, N:o 7. With 53 

 plates. Stockholm 1874. 



