„2 ECHINOIDEA. II. 



meridostenious — and this is the onh' reason which Loven can adduce for maintaining the whole 

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

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

 Ponrtalesiæ are meridosternons (de Meijere also agrees with this); the sternnm of the Pourtalesiæ is 

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

 lesiæ to the UrcchinidcE and Aiiaiichytidcr cannot then be doul:)ted either, and the systematic position 

 of the Pourtalesiæ as an extreme development of the AiuuicliyHdiT seems be\ ond doubt. 



24. Echinosigra (Pourtalesia) paradoxa "Mrtsn. 



PI. VI. Figs. 3—6, 17 — 21. PI. VII. Hg.s. 5, 10, 16, iS. PI. XI. Kigs. 2—3, 5—6, 17, 21, 24—25, 27—29, 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 Pourtalesiæ. 

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

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

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

 the test is in accordance with the other Pourtalesiæ, especialh- with its nearest relation, P. phiale. the 

 remarkable transformation being attained simply by the prolongation of some of the piates, mainly a 

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

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



The test (PI. VL 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 vviden and proceeds to the end of the «tail . 

 In the auterior, headlike widened end is the invagination characteristic of Pourtalesiæ; it is rather 

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

 edge of the invagination. — The head continnes posteriorly into a long and slender neck, highh' 

 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 fle.xible test, like Pilcviafechhms 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 having rooni onl\' for the æsophagns. 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 b\- a rather broad fasciole. The abactinal keel is not produced over the anal 

 ^rea, 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 PL \'I the shape of the test becomes 

 somewhat transformed with age, mainly by the body growing comparativeh- 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 faet probably of no small 



