8o 



SCIENCE-GOSSIP. 



L. Caerfai group .. 114,405,000 — 114,150,000 

 (The lowest bed is a conglomerate, and the 

 earliest Cambrian in this district.) 



Mid. Caerfai group 



Up. 



L. Solva group 



Mid 



Up 



L. Menevian . . 



Mid 



Up. „ 



114,150,000 — 114,100,000 

 1 14, 100,000 — 1 13,600,000 

 113,600,000 — 113,450.000 

 113,450,000 — 112,700,000 

 112,700,000 — 112,550,000 

 112,550,000 — 112,350,000 

 112,350,000 — 112,100,000 

 112,100,000 — 112,000,000 



and so on. The supposed correlation of any 

 number of sections stratigraphically or chronolo- 

 gically can, in either of these ways, be easily seen. 

 Thus, in this' particular case, the logical inference 

 is that possibly the conglomerate at the base of the 

 L. Caerfai group in Pembrokeshire may be the 

 equivalent in time of the Prospect Mountain lime- 



stone at Eureka, answering in position to the 

 asterisk shown on the chart, as also with any other 

 stratum assigned to the same position in other 

 sections. 



If the method here suggested is a sound one, it 

 would enable the geologist to correlate beds with 

 the most precise definiteness, even although they 

 may be located in different continents, present the 

 widest lithological difference in character, or belong 

 to the most diverse conditions. It has generally 

 been considered almost impossible to correlate 

 definitely and minutely the beds of different con- 

 tinents, conglomerates with limestones or marine 

 beds with fresh-water beds, but I believe this can 

 be done with as near an approach to accuracy as 

 is ever attained by the palaeontological methods 

 ordinarily used. 



4, Coaper Road, Acton; March 26, 1894. 



LARV^-NYMPHS OF BRITISH DRAGON-FLIES. 

 By W. H. Nunney. 



(~\F all insect larvae, those of butterflies and 

 ^^^ moths are the most easily recognisable ; but 

 the larvae which are treated of in the present essay 

 are by no means difficult to allot to their respective 

 species. The perfect forms of the more common 

 species are well known, even to non-entomological 

 iodividuals. The larvae, however, remain unknown, 

 except to a few specialists. Of these, Messrs. 

 Hagen, Cabot, Roster, McLachan and Dufour, 

 have bred and compared many species, and are 

 the only writers who have described the larvae 

 scientifically, the last-named being the first to 

 attempt a tabulation of the characters of some 

 larvae-nymphs, as he happily calls them. It is, 

 however, to Frederick Brauer that we are the most 

 indebted for the material from which descriptions 

 of many larvae have been made. Much of the 

 economy of certain species has been worked out 

 by the earlier authors, Reaumer, Roesel, 

 Swammerdam, Degeer, Drury, Donovan, Evans, 

 and some others of minor note. The various 

 authors who have treated the subject popularlv 

 it is needless to mention, their work being for 

 the most part unsatisfactory. The embryology 

 of one or two species has been described and 

 figured by Brandt, Balfour and Packard. Certain 

 species occur in early geological strata, the 

 Gomphina and Agrionina being in greater numbers 

 specifically. 



My intention in this article is to give a brief 

 sketch of the habits of the larvae and nymphae, 

 together with as good recognitional characters as 

 are possible with our present knowledge. Un- 

 fortunately, the specimens in our National Museum 

 are unnamed, and, although few in number, are 



scattered in different drawers, and mixed with other 

 insects. 



The larvae and pupae of the dragon-flies are so 

 similar in appearance, inter se, as to have gained 

 for themselves the semi-popular titles of larve- 

 nymphs and puppen-larve, with the French and 

 Germans, and their further likeness to the perfect 

 insects, minus wings, has resulted in the term 

 isomorphus being given to them by men of science. 

 The true larval form exists only for a very short 

 period, and the^insect, after undergoing but one or 

 two moults, attains wing-sheaths, an evidence of its 

 transition to the pupa stage. This latter form 

 occupies the greater portion of the creature's 

 aquatic life. The pupa is, in consequence, far more 

 commonly captured than the larva, and is usually 

 described under the latter name. In the pupal 

 state, to the description of which, it being the more 

 interesting and better known, I shall confine 

 myself, the insect usually remains slowly growing 

 from eight to eleven months ; some species, how- 

 ever, are said to require two or three years in 

 which to arrive at maturity. During this period 

 of growth numerous moults occur. The cast skins 

 may frequently be seen floating on the water, and 

 should be taken whenever possible, as they are 

 useful material for study and comparison of 

 species. 



The sex of these creatures is differentiated very 

 early in the larval state, and the sexual characters 

 are easily distinguishable. On the ninth segment 

 below in the female are four peculiar leaf-like 

 processes, which are metamorphosed into the 

 vulvar-scale in the female imagos ; in the male 

 these processes, when present, are atrophied, 



