EASTMAN : CARBONIFEKOUS FISHES FROM THE CENTRAL WEST. 171 



by Hancock and Howse over thirty years ago,^ and more recently by Jaekel,^ 

 the latter author reatfirniing the correctness of his predecessors' conclusions. 

 According to these writers, the strongly reflected, scoop-like extremity of the 

 crown served for a cutting-margin, and the pavement-like, rugose, or imbricated 

 portion as a triturating surface. This interpretation appears plausible enough 

 in the case of some species, but must not be supposed to be of universal appli- 

 cation amongst Petalodonts, very different conditions having existed in certain 

 other genera. Teeth of Petalodus and Petalorhynchus have been found in 

 successional series, the crowns overlapping and more or less erect (c/. Text-fig. 4), 

 but it is not known how many of such series constituted the complete dentition. 

 In these genera, as in Janassa, the median series are larger than the rest and 

 bilaterally symmetrical, those of the lateral series more or less oblique. It was 

 long ago observed by Hancock and Howse ^ that Petalo- 

 dus was provided with both symmetrical and oblique 

 rows of teeth, and that examples had been "found 

 lying in regular order, as if forming a portion of a 

 vertical row." Consequently we must express ourselves 

 as at variance with Jaekel's conjecture that the teeth of 

 Petalorhynchus probably represent the symphysial series 

 of Petalodus, their disparity in size and discordant dis- 

 tribution clearly entitling them to recognition as distinct 

 genera. There are also excellent reasons for dissenting 

 from his proposed union of Petalodus and Ctenoptychius, 

 and from his theoretical association of the lin-spines 

 known as Stichacanthus and Physonemus (including 

 Xystracanthus and Batacanthus), with Polyrhizodus and 

 Petalodus respectively. Not only do the facts of dis- 

 tribution militate with this last assumption, but the 

 absence of fin-spines in Janassa renders it extremely improbable that such 

 defences were present in other members of the same family. 



It is inferred by Hancock and Howse from the fineness of the cutting-edge 

 in two species of Janassa that the food must have consisted of soft material. 

 They state of /. bitununosa that " the scoop-like cutting-margin is certainly 

 much used, for it is almost always greatly worn in a regular manner ; only in 

 one instance have we seen it a little broken. It would be an efficient instrument 

 in cutting vegetable substances, and these might afterwards require the aid of 

 the crushing-disk." The presence of a carbonaceous mass in the abdominal 

 region of certain specimens also suggests to them herbivorous habits. Jaekel, 

 on the other hand, in discussing the probable food of the Permian J. bituminosa, 



Fig. 4. 



Petalorhynchus psittuci- 

 mis (M'Coy). Lower 

 Carboniferous; Ar- 

 magli, Ireland. Nat- 

 urally associated 

 series of seven teetii. 

 Outer face, X y- 



1 Hancock, A., and Hotvse, R., On Janassa bituminosa, Schlotheira. Ann. Mag. 

 Nat. Hist. (4), VoL V., 1870, p. 47, PI. II., III. 



2 Jaekel, 0., Ueber die Organisation der Petalodonten. Zeitschr. deutsch. geol. 

 Ges., Vol, LI., 1899, p. 258. 



3 Loc. cit., p. 61. 



