CATALOGUE OF ECHINIDA. 
37 
shell, and especially in the ambulacra being larger, more per- 
fect, and in the pores of the ambulacra being united in pairs 
by a cross groove. It differs from the fossil genus Pygaulus 
in the vent being inferior, intermediate between the mouth 
and edge, and transverse. 
This genus is named after Dr. Morton, the historian of North- 
amptonshire, who first attempted to arrange the fossil Echini 
into generic groups. 
1. Mortonia Australis. 
Elliptical, depressed, rather acute in front, rounded behind, 
under side concave near the mouth and vent ; vent large, 
oblong, transverse, in the centre between the mouth and hinder 
margin. 
Fibularia Australis, Desm. Tab. Syn. 240. 
Echinocyamus Australis, Agassiz <$f Desor, 1. c. 140. 
Mortonia Australis, Gray, Ann. <Sf Mag. N. H. 1851. 
Hab. South Sea. 
a, b, c, d. Without spines ; Australia. 
b. Mouth without any oral ambulacra , and not edged . — 
Echinobrissina. 
4. Echinobrissus. 
Shell thin, angular, squarish, wider behind ; anus superior, 
on the back of the shell, often lodged in a more or less deep 
posterior dorsal groove ; mouth five-sided, without any raised 
edges or stellate oral ambulacra. 
Echinobrissus, Breynius ; Gray , Ann. Phil. 1825, x. 7. 
Nucleolites, Lamk. Hist. ; Blainv. D. S. N. lx. 168 ; Agassiz & 
JDesor, Ann. Sci. Nat. 1847, 153. 
1. Echinobrissus recens. % 
Shell squarish ; dorsal groove distinct. 
Nucleolites recens, M. Edwards ; Cuvier, Beg. Animal, Badiata , 
t. 14, f. 3 ; Agassiz & JDesor, 1. c. 153. 
Hab. N. Holland. Quoy & Gaimard. Mus. Paris. 
Section 2. Apomesostomi, mouth excentric ; jaws none ; shell 
thin, bilateral, with a posterior dorsal groove. 
Plagystoma, Latr. Fam. B. A. 553, 1825. 
Echinides excentrostomes, Blainv. D. S. N. lx. 182, 1830. 
Pleurocysti apomesostomi, Van Phelsum. 
Apomesostomi, Klein , Echin. 38. 
