IIQ rilE nEPTllS OF THE SEA. [chap, m. 



tubercle; and the two posterior ambulacra, with 

 their ocular plates, meeting at another point and 

 forming a kind of secondary apex. The fifth genital 

 plate is obsolete. The specially interesting point is 

 that, while we had so far as we were aware no living 

 representative of this peculiar arrangement of what 

 is called ' disjunct ' ambulacra, we have long -been 

 well acquainted with a fossil family, the DysasteridcB, 

 possessing this character. Many species of the 

 genera Dysaster, Agassiz, Collyrites, Desmotjlins, 

 Metaporhinns, Michelin, and Grasia, Michelin, 

 are found from the lower oolite to the white chalk, 

 but there the family had previously been supposed to 

 have become extinct. 



The next attempt was one of our very few entirely 

 unsuccessful hauls, the dredge coming up empty. 

 This we attributed to an increase of wind and swell, 

 and conseqaent drift on the vessel, which seemed to 

 have prevented the dredge from reaching the ground. 



We devoted the morning to a series of temperature 

 soundings at intervals of 50 fathoms from the surface 

 to the bottom, and this we accomplished in a very 

 satisfactory manner, with results which will be fully 

 discussed hereafter. After a rapid descent for the 

 first 50 fathoms the next 150 fathoms maintained 

 a high and a tolerably equable temperature, and 

 there was then a rapid fall between 200 and 300 

 fathoms, the thermometer at the greater depth indi- 

 cating 0° 0. Prom 300 fathoms to the bottom the 

 temperature fell little more than a degree. " Thus 

 the entire mass of water in this channel is nearly 

 equally divided into an upper and lower stratum, the 

 lower being an AitHc stream of nearly 2,000 feet 



