ECHINOIDEA. II. 



young stages, I cannot give, since, unfortunately, no quite small specimens are found among the pres- 

 ent material. Ou the other hand, I do not doubt, as does Loven (Op. cit p. 22), that the young ones 

 will be found some day. Since we have found quite young specimens of Hemiastcr expergitus (see 

 below), a species much more rarely met with than Pourtalesia Jejfreysi, it seems not improbable that 

 we shall some day also have the good fortune to meet with the young Pourtalesia. 



The general form of the test is well described by Loven (Pourtalesia. p. 6); there is, however, 

 some variation, as pointed out by Michailovskij (Echinod. d. Jermak p. 163). Some specimens are 

 rather short and broad and with short anal rostrum, others are rather flattened; also deformities occur 

 not very seldom, with irregular depressions or with the posterior end awry (PI. V. Fig. 14), the supra- 

 anal prolongation turning to one side, the anal rostrum to the other. Also the anterior end may be 

 unequally developed, the one side projecting in front of the other. The Figures 19, 21, 23. PL V 

 represent a specimen in which the spines are uncommonly well preserved; the two side-views, PI. V. 

 Figs. 13, 18 show how different the outline in profil may be. (See also Michailovskij. Loc. cit.) 

 The species reaches a considerable size; the largest specimens at hand are up to 58""" in length. 



Wyv. Thomson states (Op. cit. p. 749) that the test is so remarkably thin that it will scarcely 

 bear its own weight*. I do not find the test of this species so very fragile; on the contrary, I find it 

 almost stout for a deep-sea species. It deserves to be noticed that among the Ingolf material there 

 are several old tests (St 113 and 117), which have evidently been partly or completely embodied in 

 the bottom deposits (they were full of mud); most of them are quite uninjured. On one of these tests 

 was found a sea-anemone, on another a sponge. - - The sutures of the abactinal lateral plates are, in 

 the larger specimens at least, generally somewhat raised, the plates themselves being somewhat con- 

 cave; this may perhaps be a structure tending to strengthen the test. 



The morphological structure of the test has been most admirably worked out by Loven. 

 There is, however, one point of interest on which my rich material enables me to make an addition 

 to our knowledge, which is of some importance, viz. the labrum and the adjoining ambulacral plates. 

 Loven finds that in P. Jeffrey st 'the labrum is quite rudimentary, only represented by a small plate 

 on the incurved edge, below the actinostome and not seen from without. The ambulacral plates I. a. i 

 and V. b. i are large and join in the median line in their whole length, whereas the plates I. b. i 

 and V. a. i are wanting (or, as Loven thinks, coalesced with the large plates I. a. i and V. b. i, which 

 are thus really compound; p. 83) a considerable difference between this species and the other species 

 examined by Loven: P. laguncula, carinata and ceratopyga, in which the labrum is distinctly seen 

 from without, separating the inner plates of ambulacra I and V; the plates I. b. i and V. a. i are also 

 developed in these species. 



This feature of P.Jeffreysi is, however, no constant one. To be sure, the labrum is often, per- 

 haps in most cases, not to be seen from without; but there is considerable variation with regard to 

 this plate. In some specimens it is seen as a very narrow plate, quite enclosed between the two large 

 inner ambulacral plates, in others it is well developed, reaching to the border of the invagination ; it 

 may even be divided into a larger outer part and a smaller inner part at the edge of the invagination 

 (PI. VIII. Fig. 10). Regarding the inner plates of ambulacra I and V there is likewise great variation. 

 I have seen one specimen in which only the plate a. i was developed in ambulacrum I, otherwise I 



