ECHINOIDEA. II. 



meridosternous and this is the only reason which L,oven can adduce for maintaining the whole 

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

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

 Pourtalesise are meridosternous (de Meijer.e also 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 Ananchytida cannot then be doubted either, and the systematic position 

 of the Pourtalesise as an extreme development of the Ananchytidce seems beyond doubt. 



24. Echinosigra ^Pourtalesia) paradoxa Mrtsn. 



PI. VI. Figs. 36, 1721. PI. VII. Figs. 5, 10, 16, 18. PL XI. Figs. 23, 56, 17, 21, 2425, 2729, 32, 42-44. 



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

 1905. p. 243. 



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

 so as to be unique in this respect even in a group containing so many curious forms as the Pourtalesise. 

 Were it not for the comparatively hard test it would by no means be easy to recognise the Echinoid 

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

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

 the test is in accordance with the other Pourtalesise, especially with its nearest relation, P. pliialc, the 

 remarkable transformation being attained simply by the prolongation of somtf of the plates, mainly a 

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

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



The test (PI. VI. Figs. 3 6, 17 21) 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 actinal keel goes from where the test begins to widen and proceeds to the end of the tail . 

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

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

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

 compressed and so very 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 Pilfinatccliimts vesica e. g. - 

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

 which is contained the intestine, the neck having room only for the oesophagus. Posteriorly the body 

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

 as usual surrounded by a rather broad fascicle. The abactinal keel is not produced 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, 18, 20 of PI. VI the shape of the test becomes 

 somewhat transformed with age, mainly by the <-body- growing comparatively higher and, espe- 

 cially, broader (thicker); in the larger specimens the ; ventral- side is rather flat (though always with 

 a median keel), the test thus keeping the natural position very easily a fact probably of no small 



