Cenfral Bohemia, by Joachim BarrandeS ' 313 



disappear in the old, as is the case in existing nature, in 

 some sessile or parasitic crustaceans. In the structure of 

 the eyes M. Barrande notices two types, — the first in Pha- 

 cops and Dalmanites, which have the cornea opaque, like the 

 other parts of the cephalic envelope, and penetrated by 

 minute holes placed in quincunx, through which the lenses 

 project ; the second, found in all the other genera, has a cor- 

 nea different from the common integument, and either smooth 

 on the surface or tuberculated over the individual lenses of 

 the compound eye. The number of these lenses varies in the 

 same species, increasing with age. It is still more variable 

 in different species, as the following very interesting table 

 will shew : — 



o • „ Lenses 



Species, in eye. 



Phacops Volborthi, Barr 14 



P. cephalotes, Corda 200 



Proetus sculptus, Barr 350 



Dalmanites Phillipsi, Barr. ... 150 



D. Hausrnanni, Brong 600 



Aeglina rediviva, Barr 750 



Bronteus Brongniarti, Barr. ... 1000 



B. palifer, Beyr 4000 



Asaphus nobilis, Barr 12,000 



Reraopleurides radians, Barr.... 15,000 



In the genus Harpes alone, simple eyes, like the stemmata 

 or ocelli of the recent articulata, appear, and only two or 

 three in number. Both these and the diverse forms of the 

 compound eyes are represented in Plate 3. From this it 

 would appear that the individual lenses are generally round 

 or hexagonal in form, and never quadrangular, as in some 

 modern Crustacea. This fact, and the smoothness of the 

 cornea, would seem to indicate that the eyes of the trilobites 

 were rather an agglomeration of simple eyes than truly com- 

 pound eyes, like those of the higher Crustacea now living. 



M. Barrande next describes the other portions of the head, 

 the cheeks, with the hypostome and epistome, pointing out the 

 value of the characters which the forms of these parts furnish 

 to the palaeontologist. We, however, pass on to the thorax, 

 and to his account of the various segments of which it con- 

 sists. As is well known, the number of these was at one 

 time thought constant in each species or genera, and this prin- 

 ciple was applied to their classification and determination. 

 The analogy of existing nature might have taught us that 

 this would probably be true only of full-grown individuals ; 



