DISTEIBUTION' OF MOLLUSCA GENERALLY. 263 



from that region enjoy a vertical range extending from less than 

 fifty to two hundred and fifty and two thousand fathoms,* which 

 would then give a very high proportion for those connecting the 

 fourth and fifth zones. 



At depths of from three hundred to one thousand fathoms mol- 

 lusks are still numerically very abundant, although the number of 

 species very rapidly diminishes. Prom an extreme depth of 3,435 

 fathoms (14,610 feet) the "Porcupine" obtained but five species, 

 and from 2,900 fathoms (17,400 feet) the " Challenger " dredged 

 only two — Semele profundorum and Callocardia Pacifica. In the 

 greatest depths the Lamellibranchiata (principally represented by 

 the families Arcadse, NuculidaB, and Pectinidae, the genera Pec- 

 chiolia, Nea>ra, &c.) appear to preponderate over the Gasteropoda, ■ 

 whose dominating forms are tectibranchs and the Scaphopoda, and, 

 among the prosobranchs, the genera Fusus and Pleurotoma. f 



The question whether the abyssal fauna is of a generally uni- 

 form type, marked by identical or representative species extend- 

 ing from pole to pole, as was first suggested by Lov6n, and subse- 

 quently admitted by Sir Wyville Thomson, still lacks the necessary 

 data required for its solution. The extremely broad or antipodal 



*"Bull. Mus. Comp. Zool.," vi., 1880. In a Bupplemental note only 

 fifty-one species (out of four hundred and sixty-twj) are recorded whose 

 range covers both the littoral and the abyssal zones, thereby reducing the 

 ratio to eleven per cent. 



t The researches of Edgar Smith and Boog Watson upon the Mollusca 

 obtained by the " Oialleuger" reveal some very remarkable instances among 

 this group of animals of ready adaptability to the most varying conditions of 

 depth, and of discontinuous habitation. Silenia Sarsii was dredged about 

 1,100 miles southwest of Australia in water of 1,950 fathoms, and again off the 

 mouth of the Eio de la Phita, in 2,C50 fathoms ; Vcrticordia Deshayesiana, 

 found off Pernambuoo in water of 350 fathoms, was also dredged off Cape 

 York in 155 fathoms ; Petrioola lapicida, a ivell-known West Indian form, 

 recurs off the North Australian coast i seven fathoms) ; and Nuculina ovalis, a 

 fossil of the Suffolk Crag, reappears in the waters of the Cape of Good Hope 

 in twenty fathoms. Venus mesodesma, a shore species, descends to 1,000 

 fathoms, while the range of Lima multicosta extends from two to 1,0V5 fathoms, 

 and of Area pteroessa from 890 to 2,050 fathoms (the West Indies and the 

 North Pacific, respectively). The total number of lamellibranch species ob- 

 tained in depths under 100 fathoms was 38i ; in depths between 100 and 500 

 fathoms, 148 ; between 500 and 1,000 fathoms, 24 (ten stations) ; and between 

 1,000 and 2,900 fathoms (thirty-three stations), 70. (" Challenger" Eeports, 

 "Zoology," xiii., 1885.) 



