296 ESCHARIDyE. 



striking difference of type as to call for the creation of a 

 distinct group. The primary orifice ranges iii the different 

 genera from semicircular (which seems to be the simplest 

 and most primitive form), through semielliptical and sub- 

 orbicular, to subquadrangular and horseshoe-shaped. 



A striking characteristic of the family is the develop- 

 ment of a secondary orifice under various modifications, 

 or the elevation of the peristome into a mucronate process 

 in front. The dentate lower margin is also found only in 

 this group, and occurs in more than half the genera com- 

 posing it. In no case is there a sinus on the inferior 

 margin of the primary orifice, nor is there any trace of 

 the special pores which occur amongst the Microporellidce 

 and Porinida. 



The erect habit of growth is common in this family. 

 It contains no less than eight British species which would 

 rank under the genus Eschara (auctt.) . 



It may be divided into three principal sections : i. 

 Species with a simple primary orifice only : Genera Le- 

 pralia and Umbonella. ii. Species with a secondary orifice 

 differing in form from the primary : Genera Porella, 

 Escharoides, Smittia, Phylactella. iii. Species with a mu- 

 cronate elevation of the peristome: Genera Mucronella, 

 Palmicellaria, Rhynchopora. 



The difference between the genus Smittia in the second 

 of these sections and Mucronella in the third lies really, as 

 I have noticed elsewhere, in the extent to which the se- 

 condary development of the peristome is carried. In 

 Umbonella verrucosa, amongst the simpler forms, the rising 

 which supports the avicularium is, I believe, the equiva- 

 lent of the secondary chamber below the primary orifice 

 in Porella*. In Lepralia Pallasiana and canthariformis , 



* We may also trace a connexion between the latter and such a form as 

 frmittia Landsborovii. 



