the Morjylwlogij of the Blastoidea. 221 



Ambulacra narrow in all but one species, not greatly de- 

 pressed Avithin the radial sinuses. Side plates lying actually 

 on the lancet-plate, and usually hiding it from view. Spira- 

 cles usually large, and, as well as the hydrospires, con- 

 structed like those of Pentremites. Anal aperture confluent 

 with the two spiracles at its sides, to form a common anal 

 spiracle. 



Obs. The name PentremitldecC^ was proposed by D^Orbigny 

 in 1849 for two Devonian Blastoids from Spain, which he 

 believed to be peculiar in having a calyx composed of but 

 two rows of plates, the basals and radials. Romerf showed, 

 however, that the two species in question, P. Paillettij d'A. 

 & de Vern., and P. Schulfzii, d'A. & de Vern., are provided, 

 like other Blastoids, with the third row of plates or orals. 

 On these grounds, therefore, Pentremi'tidea has not been 

 adopted by later writers on this interesting class. It appears, 

 however, that P. Pailletti, in common with a limited number 

 of other forms, possesses peculiarities of calicular structure 

 which separate it at once from Pentremites as understood by us ; 

 and we propose, in consequence, to rehabilitate d'Orbigny's 

 name for such species. 



It may be contended that an entirely new name would 

 have been preferable in this case. From the fact, however, 

 that d'Orbigny's types, although unknown to him, happen to 

 possess the chief points of structure on which we propose to 

 separate the genus from Pentremites^ we prefer to adopt his 

 name rather than unnecessarily burden science with a new one. 



Pentremitidea has a more slender and elongate calyx than 

 Pentremites^ or else one approaching in outline to that of 

 Oro2jhocrimis. These apparently dissimilar forms possess, in 

 common, a slender base, narrow ambulacra, and oral plates 

 entirely limited to the suminit. The side plates of the ambu- 

 lacra lie directly on, and in a great measure cover, the lancet- 

 plates, except in one species. Such characters are diametri- 

 cally opposed to those of Pentremites^ when restricted (as we 



* It may be well to state here that, in a pajier read at the York meet- 

 ing of the British Association in September I8S1, one of ns referred to 

 the g-enus Pentremitkled and to P. Paillctf i a^i having the hydrospiral slits 

 more or less concealed by the ambulacra, but partially visible at their sides. 

 We have since found, however, that a species presenting these characters 

 (which will be described under the generic name Phmiusclmma) has been 

 hitherto confounded with the true Penironitidea Pailletti, from th(> same 

 locality, which it somewhat closely resembles in external form. 



This fact seems to have escaped the notice of the donors of these 

 fossils to the national collection, both types sharing in a common labels 

 " Pentremitidea Pailletti.'^ 



t ' Monographie der Blastoideen,' Berlin, 18.52, p. 49. 



