io 6 ECHINOIDEA. II. 



Though no more recent species of Hemiaster have been described ( — except the A batus-species 

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

 the Periaster tenuis 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, especially on PI. 104, one is struck by 

 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 Hemiaster bufo, as pointed out by Agassiz, but also the total want of a latero-anal 

 fasciole, tend to show that it is really a Hemiaster. 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 H. expergitus. Also the pedicellarise point decidedly towards 

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

 Museum. The globiferous pedicellarise resemble those of H. expergitus. though more coarse (PL XV. 

 Fig. 33), the terminal opening is rather wide and surrounded by 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 pedicellariae not larger than the small form may be found, which shows that they are, 

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

 the triphyllous pedicellarise do not differ from those of expergitus. Spicules as in expergitus. 



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

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

 with referring it to the genus Periaster. Through 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, H. expergitus (incl. gibbosus). 



It may be appropriate to give in this connection some remarks on Periaster liniieola, the only 

 other recent species hitherto referred to the genus Periaster. 1 - - The tubercles along the anterior am- 

 bulacrum increase in size towards the apical system, the largest tubercle and longest spine being that 

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

 represented in a sufficiently detailed manner in the otherwise beautiful Figure 6. PI. XXVI of the 

 <sBlake»-Echiui) is said in the Panamic Deep-Sea Echini; p. 211 to be //emiasfer-\ihe, though it has 

 only two genital pores. In the specimens in hand the apical system is not very Hemiastcr-Wke; it is 

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

 the case in the figure quoted of the Blake >-Echini, though the sutures are not distinct. This species 

 is thus not in accordance with the diagnosis of the genus Periaster given by Pom el (Classif. me- 



1 In A. Agassiz and II. Lym. Clark: Preliminary Report on the Echini collected, in 1902, among the Hawaiian 

 Islands (Bull. Mus. Comp. Zool. L,. 1907), a new species of Periaster, P. maxiiiuis, is described (p. 259). 



