CRUSTACEA. 



77 



genus (Daphnid) a "smooth, undivided cornea protects and 

 transmits the rajs of light to an aggregation of small ocelli, "* 

 or eye-specks; while in a fossil species (Asaphus caudatus, 

 Fig. 5 1 ) we have an example of the cornea itself being divided 

 into at least 400 compartments, each supporting a circular 

 prominence, the whole being so arranged that where the dis- 

 tinct vision of one ceases, that of another begins. 



Among the crustaceous animals now extinct, but whose 

 remains are found in some parts of England and Ireland, 

 and in other countries, is one tribe which, from the three 

 longitudinal divisions of which the body is composed, is known 

 TRILOBITES.f 



Fig. 51. 



Fig. 52. 



by the name of Trilobites (Figs. 51, 52). In these fossils, 

 one of which has been mentioned in the preceding paragraph, 

 the compound structure of the eyes is so well developed and 

 preserved, that we are enabled to compare it with that of 

 existing species. This circumstance happily suggested to the 

 very Rev. Dr. Buckland a train of reasoning respecting ' ' the 

 condition of the ancient sea and the ancient atmosphere, and 

 the relations of both of these media to light, " which furnishes 

 so admirable an example of the manner in which knowledge 

 in one department throws light upon researches in another, 

 that we give the passage in full. 



"With respect to the waters in which the Trilobites J main- 

 tained their existence throughout the entire period of the 



* Owen, page 175. 



Fig. 51 Asaphus caudatits. Fig. 52. Calymcne Blumenbuchii. 

 Bridgewater Treatise, vol. i. page 401. 



