ECHINOIDEA. II. 

 72 



meridosternous — and this is the only reason which Lo\en can adduce for maintaining the whole 

 of the Pourtalesi£e as amphisternons. As far as I can see there cannot be the slightest doubt that 

 Lambert lEtudes morphol. sur le plastron des. Spatangides p. 93) is right in maintaining that the 

 Pourtalesiffi are meridosternous (de Meijere ako agrees with this); the sternum of the Pourtalesise is 

 not a compound plate, representing 5. a. 2 — b. 2, but a single plate, viz. 5. b. 2. The affinity of the Pourta- 

 lesise to the Urechinida and Anancliytidcr cannot then be doubted either, and the systematic position 

 of the Pourtalesiie as an extreme development of the Anaiirhytida- seems be\ond doubt. 



24. Echinosigra (Pourtalesia) paradoxa Mrtsn. 



IM. VI. Fig.s. 3-6, 17-21. PI. VII. Figs. 5, 10, 16, iS. PI. XI. FiKs. 2-3, 5-6, 17, 21, 24-25, 27—29, 32, 42 44- 

 Th. Morten sen. Some new species of Echinoidea. Vid. Medd. Naturh. Foren. Kobenhavn 



1905. p. 243. 



The shape of the test of this species is very peculiar, highh' deviating from the usual form, 

 so as to be unique in this respect even in a group containing so man\- curious forms as the Pourtalesia;. 

 Were it not for the comparativeh hard test it would b\- no means be eas\- to recognise the Echinoid 

 in this disguise. It is, indeed, an almost quite natural thing to speak of a head, neck, body and tail 

 in this species, especiall\- in the largest specimen. Nevertheless, it is easy to see that the structure of 

 the test is in accordance with the other Pourtalesia;, especially with its nearest relation, P. p/iialr. the 

 remarkable transformation being attained simply b\- the prolongation of some of the plates, mainly a 

 few of the iimer ones in the bivium, of those of the posterior paired interambulacra and an augmenta- 

 tion in the number of dorsal plates of the posterior interambnlacrum. 



The test (PI. VI. Fig.s. 3— 6, 17 — 2i| is very elongated and slender, compressed, distinctly keeled 

 above and below ; the abactinal keel is distinct in the whole length, from the head to the anal 

 area; the actiual keel goes from where the test begins to widen and ]uoceeds to the end of the «tail . 

 In the anterior, headlike widened end is the invagination characteristic of Pourtalesise; it is rather 

 sliort only about a seventh of the whole length. The front end makes onh" a rather narrow upper 

 edge of the invagination. — The head continues posteriori}- into a long and slender neck, highl>- 

 compressed and .so verv fragile that it is quite remarkable that it is not broken in two of the speci- 

 mens. One cannot help thinking that it must be rather unpractical and dangerous to have such a 

 fragile neck and that it would be more safe to have a flexible test, like Pilcmatcchimis vesica e. g. — 

 The posterior part of the test is much higher and broader than the neck , forming the «body , in 

 which is contained the intestine, the neck havmg room oul\- for the (L-sophagu.s. Posteriorly the body 

 narrows into a rather long and narrow anal snout simulaliug a tail; it bends a little upwards, and is 

 as usual surrounded by a rather broad fa.sciole. The abactinal keel i> not jiroduced over the anal 

 area, which is oval, not much sunken. The test is rather transparent, the largest specimen brownish, 

 the smaller ones lighter, almost colourless. 



As is seen on comparing figures 3 — 5 and 6, iS, 20 of PI. \'I the shape of the test becomes 

 somewhat transformed with age, mainly b\- the t)od\- growing coiujjaratively higher and, espe- 

 cially, broader (thicker); in the larger specimens the \entral side is rather flat (though alwa\ s with 

 a median keel), the test thus keeping the natural position very easily — a fact probal)l\ of no small 



