J. W. GREGORY — RELATIONS OF ECHINOID FAUNAS. 105 



can Cretaceous. It therefore appears that the gradual differentiation of the echi- 

 noids of the two areas, which commenced in the Cretaceous, had gone on until the 

 faunas appear strikingly different. 



Until a detailed revision of the American Eocene species lias been undertaken it 

 is perhaps not advisable to carry the comparison further ; but the following notes 

 on the synonyms of a few of the species appear necessary in order to tender intel- 

 ligible the use of some of the above-quoted generic terms. This is especially neces- 

 sary in the case of the genus Mprtoma and its allies. This genus was founded by 

 Desor in his "Synopsis des echinides fossiles." The diagnosis was well drawn, 

 obviously from specimens. The only species given was named M. rogersi, and a 

 reference given to Dr. Samuel Morton's figure of Scutella rogersi. This was unfor- 

 tunate, as Morton's species is a true clypeastroid, with twinned margins, and be- 

 longs to the genus Echinanthus (Leske non Breynius). The species which Desor 

 actually described was ths Scutella quinquefaria of Say. Desor's mistake has led to 

 great confusion, and the names are applied very differently in different American 

 collections. In many cases Mortonia is regarded as synonymous with Periarchus, 

 but this genus seems worthy of recognition. The type species isS. alius, Conrad, but 

 I have not been aide to see the type of this species. The common species, S. 

 pilt us-sinensis, is, however, a good example. The names, therefore, accepted by the 

 writer for this group are : 



Mortonia rogersi, Desor non Morton. 



Echinanthus quinquefaria (Say). 



Periarchus alius (Conrad). 



Another thin, flat form, in which a change of nomenclature seems necessary, is 

 the Sismondia marginalis, Conrad. The type of this is in the Academy of Natural 

 Sciences, and with its smaller ally, S. plana, Conrad, must he transferred to Monos- 



tychia. 



The Miocene Faunas. 



The Miocene echinoid fauna of the mainland of America is numerically smaller 

 than that of the Eocene and < >ligocene, but it gains considerably in size if the "West 

 Indian species be included. Most of the echinoidea described by Ravenel and 

 Tuomey from South Carolina, and referred by them to the Pliocene, must also be 

 referred to the Miocene. < >n the other hand, some species from the western states 

 usually referred to this system seem to be Pliocene or Pleistocene, and are the com- 

 mon living species ; thus some of the specimens labelled Scutella striatula, Rem., 

 really belong to the living Echinarachnius excentricus. Some of the species referred to 

 the West Indian Miocene seem also to be of later date, such as the Ehynchopygus 

 guadaloupenm, Mich., a synonym df R. caribbsearum. 



Taking, then, the Miocene echinoid fauna with these additions and restrictions, 

 we find it to present a remarkable resemblance to the Miocene echinoids of 

 the Mediterranean basin. Tins resemblance is established (1) by the presence 

 of several species common to the two faunas — e. </., Cidaris meliterms, ScMzaster 

 parkinsoni, and Schizaster scellse ; (2) by the fact that other genera are represented 

 by closely allied apecies, as in the case of the Maltese and Jamaican specie- of 

 Heteroclypem ; and (3) by the presence in both of genera with a very restricted dis- 

 tribution — e. !/., Agassizia. 



Professor Alexander Agassiz, in his interesting accounl of the origin and affinities 

 of the long existing Wesl [ndian echinoid fauna, has argued that the fact that so 



XIV lii ii. oi'.i, Soi , Am \ "i . 3 t-'ii 



