2 THE VOYAGE OF H.M.S. CHALLENGER. 



Chiton. The third example of this genus collected by the Expedition is Leptochiton 

 herguelensis, n. sp., from Royal Sound, Kerguelen, at a depth of 60 fathoms (bottom 

 temperature 40°). Leptochiton cancellatus, Sow., which is a coast and comparatively 

 shallow water form of Northern Europe, being found down to a depth of 300 fathoms off 

 Lofoten, extends from Vigo, Spain, to Greenland, and off the Canadian coast in the North 

 Atlantic down to 300 fathoms. Dall also dredged it at Alaska in from 6 to 100 fathoms. 

 Thus Leptochiton may claim to be an Arctic or north temperate genus, which also ranges 

 to great depths, at least in the North Pacific. 



Another Leptoid genus, Ilanlcyia, also appears to flourish better in colder waters. 

 Hanleyia debilis, Gray, is found in British seas northwards. Sars dredged it near North 

 Cape in from 25 to 300 fathoms, and the U.S. Fish Commission in Massachusetts Bay, 

 from 38 fathoms. Hanleyia mendicaria, Migh. and Ad., occurs at Casco Bay ; Grand 

 Manon ; Portland Harbour, Maine, the deep water specimens being much larger than those 

 from shallow water. Hanleyia (1) abyssorum, Sars, comes from 150 to 200 fathoms off 

 Bergen, Norway. Hanleyia tropicalis, Dall MS., is a large and beautiful species from 

 128 fathoms, Sand Key in the Gulf of Mexico (Dall). The allied genus Hemiarthrum 

 has only one species, Hemiarthrum setulosum, Carp., which the Expedition collected from 

 the shore of Kerguelen Island. 



Of other Chitons from moderately deep water the following maybe noted: Chceto- 

 pleura apiculata, Say, from 100 miles S.E. of Martha's Vineyard, Mass., at 210 fathoms 

 (bottom temperature 40°-5 F.), Dall, Proc. U.S. Nat. Mus., 1882, p. 410. The Expe- 

 dition dredged Lepidopleurus dorsuosus, n. sp., at 310 fathoms off Prince Edward's Island, 

 and Lepidopleurus dallii, n. sp., off South Chili from 400 fathoms (bottom temperature 

 46°"5) ; Plaxiphora carniichaelis, Gray, 345 fathoms, South-West Chili (bottom tempe- 

 rature 46°), and Plaxiphora simplex, Carp. MS., was collected on the shore and dredged 

 from 110 fathoms at Tristan da Cunha. Several species of the genus Trachydermon and 

 other genera occur at low water and at moderate depths down to about 100 fathoms. 



The general distribution of the shore Chitons is now fairly well known, and the speci- 

 mens collected by the Expedition present us with no new features of interest. 



This Report, however, brings out still more prominently the necessity of a large 

 number of specimens being collected from as many localities as possible. In some groups 

 especially there is a great tendency to individual variation, so that an unnecessary number 

 of new species have been described, or, at all events, they have been so described as to 

 mask their real affinities. In order to solve the problems of Geographical Distribution, 

 and to learn the true lesson of the variability of species, it is indispensable to compare 

 large series of specimens, and, while accurately noting and tabulating varieties, to be very 

 careful about describing them as species. For the sake of convenience, well characterised 

 or locally restricted varieties may advantageously receive some varietal designation, and 

 it would be preferable if this could convey some reference to the type or central form. If 



