206 ZOOLOGY. 
leaf-like ambulacra, and the irregularly heart-shaped, often 
elongated, form of the shell, an anterior and posterior end 
being well defined. ‘Chey for the most part live buried in 
the sand or sandy mud, not moving about so actively as the 
Desmosticha. 
Of the family Spatangide the singular genus Pourta- 
lesia (Fig. 150, P. Jeffreysit 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 representative of In- 
fulaster of the Cretaceous period. P. miranda A. Agassiz 
was dredged in the Florida Straits, in about three hundred 
Fig.150.—Pourtalesia Jeffreysii, slightly enlarged.—After Wyville-Thompson, 
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. 
Spatangus 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, off the 
coast of Maine and Massachusetts, and extending from Flor- 
ida around to Norway, is Schizaster 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 faune 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 
