Miscellaneous. 73 



studied Unger's memoir treating of the subject. 5. The Silurian 

 assemblage of Fossils, elucidated with the description of a Trilobite ; 

 and then follow (6) notes on the Fossils of the Lower Silui'ian rocks, 

 (7) those of the Limjula-heds ; (8) of the Tremadoc Slate; and (9) 

 of the Llandeilo Flags ; and here we have the modern Ccelenterate 

 analogues of the Graptolite weU shown in woodcuts ; but some 

 would Hke to have their Polyzoan alUes shown also. But these are 

 vexed questions ; and altogether, among the fossils of doubtful 

 alliances Mr. Baily walks delicatelj^ ; nevertheless he might have 

 been bolder in referring more of the so-called Annelids and Fucoids 

 to crustacean galleries and tracks (as our correspondent Mr. Albany 

 Hancock indicated long ago) ; and Ribeiria (not Bibieria) might also 

 have gone over to the Crustaceans, as Mr. Salter has suggested. So 

 much for a weak point or two ; but we must add that, as this excellent 

 work is intended for amateurs and students, as well as for experts, 

 it would have been advantageous to them if the author had given 

 the meanings of the names of the genera and species. This would 

 be generally of real use in indicating the characters, conditions, or 

 history of the things named ; and it would almost always serve in 

 some way, by association of ideas, to enable the reader and thinker 

 to remember both names and things. 



A systematic statement as to the relative abundance or peculiar 

 occurrence of the several fossils figured would also greatly enhance 

 the value of this work. 



With Plate 10 we enter upon the fossils of the Bala-Caradoc 

 formation ; and we look forward with pleasure to the illustration of 

 the other principal groups of organic remains in this convenient 

 shape, which is uniform with Professor Morris's well-known ' Cata- 

 logue of British Fossils.' 



MISCELLANEOUS. 



On the Structure of the Eye in the Gasteropoda, and on the Develop- 



?nent of the Eyes in the Animal series. By V. Hensen. 

 Various authors have recently paid attention to the structure of the 

 eye in the Gasteropoda, and more especially to that of the retina ; 

 among them are MM. Leydig, Keferstein, Kjohn, Hensen, and 

 Babouchine. The iiivestigations of these naturahsts have clearly 

 shown that the retina of the MoUusca is divided into an outer and 

 an imier layer, separated by a thin stratum of pigment. But all 

 these authors do not agree as to the parts of this retina which are 

 directly impressed by the luminous waves. Some, particularly M. 

 Hensen, regard the inner layer as sensible to light ; others, on the 

 contrary, consider it to be insensible, and that the luminous rays 

 must pass through interstices of the pigment in order to irritate the 

 outer layer of the retina. 



M. Hensen indicates that, in certain theories of vision proposed 

 by the most recent authors, the pigment plays an inadmissible part. 

 Thus, for example, the heat reflected by the pigment has been ad- 



Ann. (C; 3Iaff. N. Hist. Ser. 4. Vol. i. 6 



