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NOTICES OF NEW BOOKS. 



tures, the structure and form of the eye is naturally a subject 

 of special notice, and the author combats the notion that two dif- 

 ferent types of structure are admitted in the eyes of dhTerent ge- 

 nera of Trilobites (Calymene and Olenus), expressing his opinion 

 that all of them were provided with compound eyes covered with 

 a smooth horny membrane, and that the appearance of facets in 

 certain genera is only the consequence of the absence of this mem- 

 brane. He also states that so long as eight years ago he had dis- 

 covered the type of these eyes in a living species (Branchipus 

 stagnalis), and that they were made up of four membranous coats 

 of different kinds, the outer one being horny, smooth, homogeneous 

 and transparent ; and under this another existing of a 

 different kind marked into facets and containing on 

 a clear substance little, circular, equal-sized divisions, 

 of a somewhat darker and harder material. The 

 third coat of the eye consists of egg-shaped clear 

 and very hard lenses, one of which is behind each of 

 the window-like apertures j ust described. The fourth 

 coat consists of an oblong crystalline body with a 

 tuberculated surface, which with its upper thicker 

 end embraces the pointed end of the egg-shaped 

 lenses, and is covered by a thin circumscribing mem- 

 brane. A continuation of this membrane envelopes 

 also the lenses, and is attached to the thickened border 

 of the little apertures before every lens. Behind the 

 crystalline body comes a dark pigment, the chief 

 mass of the whole eye, through which the nerves 

 pass to go to the separate ocelli and touch the base 

 of the crystalline body, so that their separation or 

 forking encloses the before -mentioned separation of the crys- 

 talline substance and the lenses, and through these passes on in 

 the same manner to the facetted membrane next in order. 



It would appear that this description is perfectly applicable to 

 those Trilobites having a smooth, horny membrane on the eye ; 

 and as in the living Articulata the number of the separate ocelli 

 rather increases than diminishes as the whole eye is smaller, while 

 the horny membrane is thinner as the eye is larger, we may un- 

 derstand that in these extinct genera the species with large, pro- 

 minent, numerously facetted eyes may not in reality possess 

 differently constituted organs of vision from those whose eyes are 

 exceedingly flat and covered with a thick horny film. 



In this short notice it is not possible to do more than allude to 

 the greater number of Dr. Burmeister's researches on the different 

 parts of the body of the Trilobite. He considers that the pro- 

 minence in the middle anterior part of the horny helmet is the 

 region of the mouth as it is in Phyllopoda, and that all affinity to 

 the Isopoda is thus widely departed from. He also alludes to 

 the number of the rings or divisions of the horny shield as a matter 

 of considerable importance in determining the position of these 

 animals in classification. 



