Wyatt-Edgell — On the Genera of Trilobites. 



15 



cites but one species of Ogygia that bears resemblance in tbis 

 respect.^ Yet with all the stamp of an Asaphus, Ptycliopyge Corn- 

 densis has the obtusely -pointed labrum common to Ogygia Buchii and 

 0. Selwynii. (See Woodcut, Fig. 2.) 



Fig. 1. Asaphus [Ptychopyge] Corndensis, Murch. Llandeilo Flags, Builth.. Fig. 2. Labrum 

 of same. Fig. 3. Labrum of Ogygia scutatrix, Salter. Fig. 4. Labrum of Asaphus tyrannus, 

 Murch. Fig. 5. Labrum of Ogygia peltata, Salter. [Drawn from a sketch, by the Author, after 

 figures upon Plates XVI., XVII., and XXII., of Mr. Salter's Monograph.] 



The question arises whether the form of the labrum is to be con- 

 sidered of such importance as alone to distinguish genera. The 

 fact of the labrum of Ogygia peltata (Woodcut, Eig. 5) (which is 

 an unmistakeable Ogygia, with narrow axis, remote fulcrum, and 

 duplicate ribs to the tail) not being symmetrical, would make one 

 think that too much stress has been laid on this point. The 

 labrum has two small auricles, and slightly resembles that of 

 Asaphus tyrannus (Woodcut, Fig. 4), Ogygia scutatrix, so nearly 

 allied to 0. peltata as to have been formerly confounded vpith it, 

 has a symmetrical, almost rectangular labrum (Woodcut, Fig. 3). 

 Thus the shape of this organ differs exceedingly in these two very 

 similar forms. It may be added that in Ogygia Sehcynii, which 

 makes the nearest approach to Asaphus of any species of its genus 

 (as here defined), the labrum is obtusely-pointed, though broad; so 

 that, if its form were to be considered a generic distinction, the two 

 last-named species would have to be separated from the first. But 

 no, it is evident that they cannot be — surely we must look to other 

 features to mark the genus ; and this being the case, Ptychopyge must 

 be separated from it to take its place as Asaphus. 



Therefore setting aside the labrum altogether, the differences be- 

 tween Ogygia and Asaphus can be summed uj) as follows : — 



1. Ogygia is not generally so convex, and iias the fulcrum farther 



removed from the axis. 



2. In it the pleural furrows extend farther, and have the appear- 



ance of incisions, not that of broad grooves. 



1 0. dilatata Dalman, a foreign species. 



