142 SPECIES NOT 



the essential optical apparatus, and placed upon 

 moveable peduncles. Ocelli or stomatu are com- 

 bined with compound eyes in the same species 

 in certain Entomostracans, as apus and limuliis. 

 A transparent speck of the integument forms the 

 cornea of the ocellus, immediately behind which 

 there is a spherical crystalline body in contact 

 with a gelatinous or vitreous humour, upon which 

 the extremity of the optic nerve expands ; a layer 

 of dark pigmentum covers all these parts, with 



the exception of the cornea." u ln the Trilobites 



the cornea presents the same sub-divided surfun- 

 as in the limulus ; and the position of the two 

 eyes agrees with that of the corresponding com- 

 pound pair in the large existing Entomostracan. 

 In the Asaphas caudatus, (a Trilobite,) the cornea 

 is divided into at least four hundred compartments, 

 each supporting a circular prominence ; its general 

 form is that of the frustum of a cone incomplete 

 towards the middle of the head, but commanding 

 so much of the horizon in other directions, that 

 where the distinct vision of one eye ceases, thut 

 of the other begins" (Lectures on the Inverte- 

 brata, p. 174-5. 1843.) 



Professor Owen, after finishing his description, 

 then quotes from Dr. Buckland, the following 

 striking and convincing remarks: 



The eyes of the Trilobites of the transition 

 rocks, and those of their nearest congeners, the 

 fossil limuli, from the carboniferous series, "give 

 information," says Dr. Buckland, "regarding the 

 condition of the ancient sea and ancient atmos- 



