82 PERMIAN FOSSILS. 



The subdivisions Helictopoda and Ancyclopoda, lately proposed by Mr. J. E. Gray, 

 are respectively represented by Palliobranchs possessing spirally-rolled, and recurvedly- 

 folded labial appendages.^ 



Sub-kingdom HELICTOPODA/ J. E. Gray. 

 Diagnosis. — " The oval arms are elongate, regularly spirally twisted when in 



"3 



repose. 



I have not transcribed the whole of the diagnosis given by Mr. J. E, Gray, as he 

 was not aware, at the time of drawing it up, that the group, contrary to his views, 

 comprised several genera, having the substance of the valves pierced with minute 

 perforations.* 



true brachiferous Palliobranch. "Although Philippi describes Terebratula detruncata (the type of Argiope) 

 as having no arms, but only cirrhi attached to the apophyses, our own examination of that animal would 

 rather go to maintain the existence of true but fixed arms ; and in the curious Orthis anomoides of Scacchi 

 and Philippi (which is the Terebratula depressa of the ' Report on the MoUusca of the ^gean), the latter 

 eminent malacologist figures and describes two perfect spiral cirrigerous arms, &c." — British MoUusca, 

 vol. ii, p. 360. The expression " spiral cirrigerous arms," — whether does it imply organs such as the spirally- 

 folded labial appendages of the Helictopods, or the recurvedly-folded homologues of the Ancylopods ? 



1 'On the Arrangement of the Brachiopoda,' by J. B. Gray, Esq., F.R.S,, in the Annals and Magazine 

 of Natural History, 2d series, vol. ii, pp. 435-9, 1848. 



2 I have reversed the order of Mr. Gray's arrangement, as I feel persuaded that all classifications of the 

 Animal and Vegetable Kingdoms should be based on the two principles— or<?er of creation, and affinity. I 

 have elsewhere alluded to these principles (vide Annals and Magazine of Natural History, vol. xiv, pp. 27 1-9) ; 

 and I regret that I am not yet prepared to draw up a classification of paUiobranchiate shells, in accordance 

 with them, or what may be termed the Chronogenic system. It may suffice for the present, however, 

 my observing, that the Helictopods appear to have been the earliest created forms of the class. It is 

 exceedingly difficult to say which genus was first created. Perhaps the most correct view is the one 

 admitting a synchronous creation of several genera ; for it is a remarkable fact, that out of thirty-four 

 known hehctopodous genera, about twenty-six have been found in the Silurian or earliest rocks. I 

 have commenced with Lingula, because it is certainly one of a few of the earliest paUiobranchiate genera 

 known, having been found in the lowest of the Silurians, at Tremadoc, in North Wales, and at Kelsville 

 (Potsdam Sandstone), in the United States. With regard to the Ancylopods, it is not so clear that any of 

 them existed in the earliest organic periods ; at least, I do not feel myself qualified to speak positively 

 regarding any, with the exception of Strigocepkalus, and this is a genus not yet known to occur in any rocks 

 below the Devonian. The ever-prevailing paucity of the Ancylopods is strikingly contrasted with the almost 

 constant profusion of the Helictopods. Neither division may be said to be abundant at the present time ; but 

 perhaps the Ancylopods were seldom (unless we except the Jurassic period) much more numerous than what 

 they are at present. 



3 Op. cit., p. 437. 



* The remainder of the diagnosis is as follows " The mantle-lobes are merely applied- to the inner 

 surface of the shell, and the substance of the valves is not pierced with minute perforations, though the 

 surface is sometimes spinulose, the spines being only formed on the edge of the shell while it is being 

 increased in size." 



