160 
CEISIXA. 
Type Species. 
Crisina cenomana, d’Orbigny. Prod. Pal. toI. ii. 1850, p. 175. 
Cenomanian : Le Mans. 
AFFUflTIES. 
The genus Idmonea was founded by Lamoiiroux in 1821 for 
a single Bathonian species. The zoarium is sessile, and consists 
of triangular encrusting branches with alternate rows of apertures, 
and frequently a well-developed selvage. Later workers on the 
Brjmzoa have used the name in a different sense. Busk described 
the whole of the Idmoniidse as erect, and d’Orbigny proposed the 
genus Reptotuhigera, in which he included the type species of 
Idmonea. Idmonea triguetra., which we are bound to accept as the 
type species of Idmonea, presents some striking differences from the 
later species usually refeiTed to that genus. It is convenient to 
adopt for them another generic name. D’Orbigny’s Crisina appears 
to he the most suitable. It was defined thus : “C’est une Idmonea, 
dont les branches sont libres au lieu d’etre fixes.” There are 
other differences, hut the character named is the most conspicuous. 
It is true that d’Orbigny subsequently proposed a fresh distinction 
between Crisina and Idmonea, using the fonner for the species in 
which there are pores on the reverse surface ; and unfortunately 
Jullien and Stoliczka have accepted Crisina in accordance with this 
later definition. But the pores in question are mere interzooecial 
depressions, of no morphological value. That these are, moreover, 
of little systematic value is rendered probable by d’Orhigny’s own 
use of them. On pi. 614 of his “ Cretaceous Biyozoa ” three species 
are figui’ed, all of which are referred to Crisina ; hut in the text of 
the volume C. cenomana (although the only species amongst those 
originally included in Crisina which is available as a member 
of that genus) is removed to Idmonea ; and the reverse face of 
C. cenomana is marked by the interzooecial depressions or pores, 
which are said to he characteristic of Crisina and to he absent 
fi’om Idmonea. 
Coelopliyma of von Hagenow was coiTectly explained by d’Orhigny 
as Crisina showing the gonoecia. Clavitiihigera, d’Orb., is a Crisina 
with short broad zoarium. 
