ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 589 



some 98 genera are found in the ' Blake ' collection. The 461 species 

 included in the last -mentioned collection comprise 210 which are 

 littoral or do not reach great depths, while 251 are ahyssal or 

 ubiquitous. These nimibers are of course approximate, and subject to 

 correction, but probably not seriously in error. 



Out of 48 species, of 44 genera, having great vertical range, 

 24 have a range of 500 to 750 fathoms ; 17 have a range of from 

 750 to 1000 fathoms; and 7 have a range of from 1000 to 1555 

 fathoms. Bearing in mind that the absolute depth of the extreme 

 range may be much greater than this, the astonishing fact is evident 

 that the same species may experience a difference, between two of its 

 stations, of the weight of nearly two miles of sea-water. The 

 possibility of this of course lies in the permeation of the soft parts by 

 the sea-water, thus equalizing the pressure. It is almost certain, 

 however, that individuals from the great depths would die if removed 

 to shoaler water, unless by extremely slow degrees. 



It is noticeable among the deep-sea forms that the sculpture tends 

 to be slight, the shell thin, pale or colourless, and in the spiral shells 

 there is a tendency to a tiobbing or denticulation of the posterior 

 edge of the whorls at the suture. To each of these peculiarities there 

 are, however, conspicuous exceptions. 



Tables are appended giving in detail the statistics from which the 

 foregoing conclusions have been drawn. 



Molluscoida. 



Timicata of the ' Challenger ' Expedition.* — Dr. Herdmann, in 

 the third part of his preliminary report, deals with the Cyntliiadce, 

 which family he re-defines as follows : — Body attached, sessile, or 

 pedunculated ; apertures four-lobed, or having less than four lobes ; 

 test coriaceous ; rarely cartilaginous or gelatinous ; branchial sac 

 folded, with internal longitudinal bars, but no papillas ; stigmata 

 straight ; tentacles simple or compound. Of the four tribes of 

 Savigny's genus Gyntlda, two disappear, one being referred to the 

 Molgulidae, and the other having been shown by E. Hertwig to have 

 been founded under a misapprehension ; the others are retained as the 

 sub-families CyntJiince and Styelince, and a third — Boltenince — -is now 

 formed for the reception of the old genus Boltenia, the new Culeolus, 

 and, probably, Cystingia. The author, as in his previous commimica- 

 cationSj gives a table to show the arrangement of the sub-families and 

 genera. 



In the genus Microcosmus, M. Helleri and M. propinquus are new ; 

 in Cynthia, C. cerehriformis, C. fissa, C. formosa, C. arenosa, C. irregu- 

 laris, G. Jiispicla, and C. coniplanata are new ; in the genus Styela ten 

 new species are described, and eleven in Polycarjya. The new genus 

 Culeolus is formed for the reception of six deep-sea species of stalked 

 simple Ascidians, which closely resemble in appearance and in many 

 points of their structure Boltenia, from which they are to be distin- 

 guished by the structure of their branchial sac. " It is merely a 

 skeleton compared with that organ in other simple Ascidians. The 



* Proc. K. Soc. Edinb., xi. (1881) pp. 52-88. 

 Ser. 2.— Vol. I. 2 B 



