19 
pralia and Schizoporella, are connected by a number of transitional 
forms not only with each other, but also with the orbicular and the 
suborbicular orifice.  AÅ character apparently of much greater clas- 
sificatory value than the form of the orifice (though it is subject 
also to a considerably variation and can be found in different fa- 
milies) is a structure which we will name the oral bow. It is an 
arched projection from the whole of the distal margin of the ori- 
fice, extending more or less deeply into the zooecial cavity, in most 
cases (f. i. im Reteporidae, Porella, Microporella) forming only a 
low internal collar, but sometimes developed into a rather large 
concave plate (Mucronella diaphana). It reaches the greatest deve- 
lopment in the species of Mucronella (ch. emend.), in Escharoides 
Sarsii and ,Porina" magnirostris, and is very well represented in 
a number of figures in Busk's Crag Polyzoa") (f. i. in Pl. IV, fig. 
4, Pl, V, fig. 8, Pl. VI, fig. 4). In the Cfeilostomata it is not unusual 
to find that characters, which as a rule are subject to variation, in 
certain families can show great constancy and therefore it is not an 
easy matter to set forth common rules for the relative value of sy- 
stematic characters. Nevertheless I have no doubt that the structural 
features, generally most to be relied on, for the distinction of fa- 
milies and genera, are the rosette-plates and the o00ecia. 
While in a work later to be published I intend to give de- 
tailed definitions of all the families of Cheilostomatous Bryozoa, 
which I have been able to study, I shall now merely confine myself 
to making a few remarks upon a part of them. 
Flustridae.. From the other families of Malacostega the Flu- 
stridæ are best distinguished by the endozooecial ocecia and the 
vicarious avicularia. In a number of species exists a feebly deve- 
loped cryptocyst. The species described. under the name of Flu- 
stra, but provided with free hypostegial ocecia (F/. nobilis, Fl. crassa 
and Fy, dissimilis) must be referred to the Bicellariidae, whereas 
2 å Monograph of the fossil Polyzoa of the Crag, Mae eGeR 
Society, 1857, London 1859. 
ar 
