CLARK: AUSTRALIAN AND INDO-PACIFIC ECHINODERMS. 113 



ficial resemblance to Opliiocoma. Such species of Pectiuura as P. septemspinosa, 

 with its greatly elongated lowest spine, and of Ophiopeza, as O. danbyi, with its 

 four, long, thick arm-spiues, indicate clearly how such a group might arise. 



The evidence afforded by the arm-spines is strongly confirmed when we take 

 into account the other morphological characters of the genera concerned. De 

 Loriol long ago called attention to the fact that in some way a serious error has 

 slipped into the diagnosis of Ophiarachna, for Lyman and others have said that 

 the genus was characterized (as is Opliiocoma) by a cluster of tooth-papillae 

 at the apex of the jaw. As a matter of fact Ophiarachna differs sharply from 

 Opliiocoma in this particular ; there are no tooth-papillae, but the teeth and oral 

 papillae are strikingly like those of Pectiuura. Another error in Lyman's diag- 

 nosis of Ophiarachna is the statement, " one or two tentacle-scales." As a matter 

 of fact there are always two tentacle-scales, and they are arranged as in Pectinura 

 and its allies, with the outer one overlapping the base of the lowest arm-spine. 

 In Ophiocoma and its allies, on the other hand, if two tentacle-scales are present, 

 the outer oue does not overlap the base of an arm-spine. 1 The internal anatomy 

 of the jaw-frames, moreover, is, in Ophiarachna, like that of Ophiopeza and quite 

 different from that of Ophiocoma (see p. 127)- In the presence of supplementary 

 oral plates and of pores between the basal under arm-plates, Ophiarachna differs 

 markedly from Ophiocoma and shows a strong resemblance to several species of 

 Pectiuura. 



In consideration of all these facts, it seems clear that Ophiarachna is a near 

 ally of Pectiuura, with the arm-spines showing an interesting parallelism to 

 Ophiocoma. Indeed it is by no means easy to point out auy character or group 

 of characters by which all the species of Ophiarachna can be readily separated 

 from all the species of Pectinura and Ophiopeza. 



Another conclusion to which my investigations have led is that the complaints 

 which have arisen from time to time in regard to the inconstancy of the difference 

 between Pectinura and Ophiopeza are well grounded, and that the two genera 

 cannot be distinguished by the presence or absence of the supplementary oral 

 plates. I have before me two excellent specimens of Pectinura maculata Verrill, 

 in one of which the supplementary plates are well developed, while in the other 

 they are wholly wanting. On the other hand, Ophiopeza fallax Peters has long 

 been known to vary greatly in this respect, some specimens showing one or more 

 of the supplementary plates. The specimens of Ophiopeza cylindrica Hutton, 

 which are accessible, are all Pectinuras in this particular, for one has one, one 

 has four, and one has five supplementary plates. Moreover in several of the 

 deep-sea species which have been referred to Pectinura, the supplementary plates 



1 Troschel (1879, Sitzungsb. Niederrhein. Gesellsch. Bonn, p. 137) gives a diag- 

 nosis of Ophiarachna in which he distinctly says " keine Zahnpapillen " and 

 " Schuppen an den Tentakel-poren." Lutken (1869, Add. Hist. Oph., 3, p. 74) 

 says, " To fodpapiller " but does not mention tooth-papillae. Lyman (1882, " Chal- 

 lenger "Oph., p. 173) says, "very numerous, close-set tooth-papillae," and " one 

 or two tentacle-scales." 



vol. lii. — no. 7 8 



