BY R. ETHERIDGE, JUN., AND JOHN MITCHELL. 313 



3. Xiphogonium, Corda, 1847. Pleurae of the pygidium unseg- 



mented ; nine segments to the thorax. 



4. Celmus, Angelin, 1878. Two pairs of glabella furrows; 



twelve thoracic segments. 

 The non-acceptance of these terms by naturalists in general 

 would seem to imply a difficulty in distributing the species under 

 them. To us, however, Phceton appears a sound section of Proetus, 

 and had any species occurred possessing its peculiar features 

 we should have felt disposed to use it. Our inclination, also, 

 would lead us to adopt Sir F. McCoy's Forbesia, were it not that 

 the characters assigned appear to be more specific than even sub- 

 generic. For instance, his F. euryceps does not show two of the 

 most important characters used for its generic separation — the 

 neck tubercles and glabella furrows. Some Proeti have segmented 

 pygidial pleurae, without the presence of genal spines, e.g., P. 

 bohemicus, Barr.*; others again possess genal spines, but no 

 glabella furrows, and a well-segmented pygidium, as, for instance, 

 P. lepidus, Barr. f ; a third species is provided with glabella 

 furrows and genal spines, but the pleurae of the pygidium are 

 almost smooth ; lastly, many species have well-developed neck 

 lobes and genal spines, without the presence of neck lobes, and 

 genal spines alone in the absence of glabella furrows, such as P. 

 Prouti, Shumard, P. Powi, Green, sp.J Under these circum- 

 stances, Forbesia does not appear to have sufficient stability to 

 form even a sub-genus. 



Proetus bowningensis, Mitchell (PI. xxv., figs. 1, la-c). 



Proetus bovmingensis, Mitchell, Proc. Linn. Soc. N. S. Wales, 



1887, I. (2), Pt. 3, p. 439, t. 16, f. 4-6. 



Sp. Char. — Body subelliptical. Cephalic shield larger than the 

 pygidium or thorax, equal in axial length to the pygidium, greater 

 than the axial length of the thorax by the width of the neck ring 

 and furrow ; limb moderately wide, the margin thickened, flattened 



* Syst. Sil. Boheme, 1852, I. Atlas, t. 16, f. 1-15. 

 t Syst. Sil. Boheme, 1852, I., t. 16, f. 28-30. 

 % Hall, Pal. N. York, 111. Dev. Fossils, t. 21. 



