io6 



ECHINOIDEA. II. 



Thoiio-h no more recent species of Hemiaster have been described (— except the ^ (^(7/»,f-species 

 wrongly referred to this genus — ) there is reason to discuss one more species in this connection, viz. 

 the Periaster tennis A. Ag. described and figured in the Panamic Deep-Sea Echini p. 209, PI. 103, 

 figs. 5_7, 104, 105, figs. 1—3. At the first glance on the figures, especialh- on PI. 104, one is struck b\- 

 the close resemblance of this species to a Hemiaster, and a study of the details of the structure of 

 the test can only strengthen the first impression. Above all the ethmophract apical system, so closely 

 like that of Ilniiiaster hiifo, as pointed out by Agassiz, but also the total want of a latero-anal 

 fasciole, tend to show that it is really a Hevtiaster. Further the elongate labrum, reaching to the 

 middle of the second ambulacral plates of the adjoining series, the condition of the petals and the 

 shape of the test, recall very much //. expergitMs. Also the pedicellarice point decidedly towards 

 Hemiaster, as I can state having examined a specimen (Albatross- St. 3398) in the U. S. National 

 Museum. The globiferous pedicellarite resemble those of //. expergitus, though more coarse (PI. XV. 

 Fig. 33), the terminal opening is rather wide and surrounded b>- teeth as in expergitus; they are, un- 

 fortunately, all somewhat broken in the only specimen found. The blade is a rather wide tube, with a 

 comparatively narrow (glandular) space continuing down into the basal part. The stalk is thick and 

 compact, but without distinct thickening or projections. The tridentate pedicellarise are of two kinds 

 of different size; the small form (PI. XV. Fig. 49) is very like that of expergitus. only the skin is much 

 thicker, especially the neck is very conspicuous; the large form (head ca. 07"'") differs from that of 

 expergitus in the outer part of the blade being more rounded (PI. XV. Fig. 4). Specimens of this kind 

 of tridentate pedicellaria; not larger than the small form may be found, which shows that lhe\- are, 

 indeed, two separate forms of pedicellarise. Rostrate and ophicephalous pedicellaria; were not found; 

 the triph)-llous pedicellaria; do not differ from those of expergitus. vSpicules as in expergitus. 



From what has here been pointed out I think it evident that this species really belongs to 

 the genus Ilrnnaster, the absence of a latero-anal fasciole especially being a character non-conformable 

 with referring it to the genus Periaster. Tlirough the prominent labrum and narrow plastron, as well 

 as through the pedicellarise and the general shape of the test (especially the outline in profil — comp. 

 PI. II. Fig. 20 with PI. 104. Fig. 3 of the <:Pan. Deep-Sea Ech.») Hemiaster tenuis (A. Ag.), as its name 

 must be, is easily distinguished from its nearest relation, //. expergitus (incl. gibbosus). 



It may be appropriate to give in this connection some remarks on Periaster liinicnln. tlie onl\- 

 other recent species hitherto referred to the genus Periaster.^ — The tubercles along the anterior am- 

 bulacrum increase in size towards the apical system, the largest tuljercle and longest spine being tliat 

 nearest the apical system, as is also the case in Hemiaster expergitus. Tlic apical system (which is not 

 represented in a sufficiently detailed manner in the otherwise beautiful Figure 6. PI. XX\'I of the 

 «Blake»-Echini) is said in the Panamic Deep-Sea iCchini p. 211 to be //eniiaster-WVc, though it luis 

 only two genital pores. In tlie specimens in hand the apical s\stcm is not ver)' Hemiaster-\ikc\ it is 

 ethniolytic, the madreporite separating also the posterior ocular plates (Fig. 21). This is evidently al.so 

 the case in the figure quoted of the <Blake»-Echini, though the sutures arc not distinct. This species 

 is thus not in accordance with the diagno.sis of the genus Periaster given by Pomel (Classif. mc- 



■ In A. Agassi/, and M. hym. Clark: Preliminary Report on tlic Kcliini collected, in 1902, aniont; the II.i\vaii:in 

 Islands (Bull. Mus. Comp. Zool. h. 1907), a new species of Periaslit; P. maximn.i., is described |p. 259). 



