BY R. ETHERIDGE, JUN., AND JOHN MITCHELL. 317 



Obs. — Proetus Rattei was figured and partially described by the 

 late Mr. Felix Ratte, and provisionally referred to P. ascanius, 

 Corda, but though the cephalic shield does to some extent agree 

 with that of this species, as figured by Barrande,* there are 

 differences which we consider sufficiently important to separate 

 the two, and therefore beg to associate with it the name of our 

 deceased friend. 



The glabella in P. Rattei is much squarer than in P. ascanius, 

 and approaches nearer to the hinder border of the limb. The 

 longitudinal furrows also are longer, extending farther forward, 

 whilst the basal lobe is smaller. At the same time, both Trilo- 

 bites possess a great development of cephalic shield exterior to 

 the glabella, and it was probably this, with the character of the 

 furrows, which caused Mr. Ratte to make the reference to P. 

 ascanius. Mr. Ratte appears to have copied Barrande's figures 

 rather than the Australian fossils before him. P. Rattei is quite 

 distinct from either of the other Australian species. 



In some respects the pygidium of P. Rattei resembles that of 

 P. decorus, Barr.,f but differs in being nearly semicircular instead 

 of sub triangular. In the great proportionate size of the limb of 

 the pygidium, our species approaches some forms of Bronteus; and 

 in the short glabella and expanded cephalic shield it resembles 

 Arethusina. On the other hand, the Proetiform characters of 

 eight thoracic segments, and eyes close to the glabella, overbalance 

 these peculiarities. 



A similar acicular ridge to that at the posterior termination of 

 the pygidium axis is also present in Proetus corycazus, \ Conrad, a 

 characteristic species of the Niagara Group. One of the most 

 peculiar features in P. Rattei, and unobserved by us in any 

 other species of the genus, is the enlargement of the pleural 

 spine of the third thoracic segment. Is this a sexual character 1 



We have detected what we believe to be the larval condition. 

 In the earliest stage the glabella is rudimentary, the eye exceed- 



* Syst. Sil. Boheme, 1852, I., Atlas, t. 15, f. 41. 



f Syst. Sil. Boheme, 1852, I., Atlas, t. 17, f. 13. 



X Hall, Pal. N. York, II., t. 67, f. 13. 



