124 ZOOLOGY. 



leaf -like ambulacra, and the irregularly heart-shaped, often 

 elongated, form of the shell, an anterior and posterior end 

 being well defined. They for the most part live buried in 

 the sand or sandy mud, not moving about so actively as the 

 Desmosticha. 



Of the family Spatangidce the singular genus Pourta- 

 lesia (Pig. 87, P. Jeffreysii Wyville- Thompson) deserves 

 notice, the species of which are bottled-shaped, with a thin, 

 transparent shell. The transition from such a form as this 

 to the Holothurians is not a very extreme one. This 

 genus, A. Agassiz states, is the living representatiTe of In- 

 fulaster of the Cretaceous period. P. miranda A. Agassiz 

 was dredged in the Florida btraits, in about three hundred 



Pig. g!.—Pourtalesia Jeffreysii, slightly enlarged.— After Wy\Tlle-'niompBon. 



and fifty fathoms, and by British naturalists in the Shet- 

 land Channel. P. Jeffreysii was dredged in six hundred 

 and forty fathoms, near the Shetland Islands. 



Spatmigus is distinctly heart-shaped, as is Hemiaster. 

 An interesting deep-sea or abyssal form not uncommon in 

 deep soft mud, at the depth of one hundred fathoms, ofE the 

 coast of Maine and Massachusetts, and extending from Flor- 

 ida around to Norway, is ScTiizaster fragilis Agassiz. 



Echinoderms range to a great depth in the ocean, and are 

 largely characteristic of the abyssal fauna of the globe. In 

 space they are widely distributed, there being but two 

 Echinid faunae on the eastern coast of the United States, 

 one arctic, the other tropical. While a large number of 

 species characterize the arctic or circumpolar regions, the 



