160 THE VOYAGE OF H.M.S. CHALLENGER. 



both De Loriol and Zittel have like Pictet assigned to the group. The characters of the 

 recent genera show a far closer affinity between the Holasteridse and the Spatangidae than 

 had been suspected from the study of the fossil species alone, and for similar resons I 

 should hardly wish to adopt the group PalcBOStoma of De Loriol characterised by the pen- 

 tagonal actinostome alone, a structural feature which in such genera as Paleopneustes, 

 Aceste, Aerope, and the like is shown to pass so insensibly into the labiate actinostome 

 of the Spatangoids as scarcely to justify us in adopting it as a character for the dis- 

 tinction of higher groups. 



*Argopatagus. 

 Argopatagns, A. Agassiz, 1879, Proc. Am. Acad., vol. xiv. p. 209. 

 This genus is closely allied to Homolampas ; it has like it a subanal fasciole 

 (PI. XXXII. figs. 2, 4), but no peripetalous fasciole ; the primary tubercles both on the 

 ambulacra and interambulacra are largest and most crowded at the ambitus; they become 

 less numerous towards the abactinal pole, and smaller though numerous towards the 

 actinostome. The odd anterior ambulacrum is not sunken, but like the others flush with 

 the test. The structure of the ambulacra is similar to that of Homolampas, but on the 

 abactinal surface the ambulacral plates are larger in comparison with the interambulacra! 

 ones than in that genus ; they are all more or less hexagonal. The primary tuliercles are 

 more numerous in the odd anterior ambulacrum on the abactinal side (PI. XXXII. fig. 1). 

 The structiu-e of the apical system is like that of Homolampas, it is compact 

 (PI. XXXII. fig. 6) ; there are four genital openings enclosing a distinct madreporic body, 

 the sutures of the genital plates are obliterated, the genitals are equally developed. 

 The five or six ambulacral suckers near the abactinal pole are more powerful with small 

 sucking disks, the other suckers rapidly becoming more slender towards the ambitus. 

 These large amlmlacral suckers (PL XXXII. fig. 6) form a rudimentary petaloid area 

 much as in embryo Spatangoids, but not by any actual petaloid arrangement of the pores. 



''Argopatagns vitreus (PI. XXXII. figs. 1-6 ; PI. XXXVIII. fig. 25 ; PL XXXIX. 

 fig. 18; PL XLI. figs. 32-35). 



Argopatagus vitreus, A. Agassiz, 1879, Proc. Am. Acad., vol. xiv.' p. 209. 



In tliis species the test is gxeatly flattened, extremely thin, transparent, of a 

 yellowish colour. The large primary spines of the abactinal region are cylindrical, 

 curved, of a silvery lustre, with distant sharp spinules along the shaft. The test is quite 

 flattened (PL XXXII. fig. 3), and at first sight reminds one when seen from above some- 

 what of Maretia. There were only very imperfect specimens of this species collected, 

 the actinal plastron was missing in all, but as far as the actinal side was preserved 

 (PL XXXII. fig. 2), the primary tubercles of that side were smaller than on the upper 

 side and clustered together near the ambitus, forming elongate triangular patches in the 



