28 CLASS IIl. GASTEROPODA. ORDER III CERVICOBRANCHIATA. 
subobliqué inclinato. Apertura septo aut lamina horizontali plus 
minusve clausa. 
In carrying out the artificial arrangement of organic nature, it is 
curious to observe with what precision the whole system is gradually 
advancing to one complete chain of affinity by the discovery of kinds. 
The further we advance in the knowledge of creation, the more ineffectual 
becomes the ingenuity of the naturalist to perfect his system of arbitrary 
division. Most of us are wisely satisfied with the existing form of clas- 
sification, esteeming it as safe and sufficient a formula as the memory 
requires in the study of this science; there are some, however, whose 
restless ambition prompts them to trace generic limits where none but 
the simplest modifications of character exist, and they delight in giving 
to every slight variation a local distinction and a name. Thus it is with 
the Crepidule and Calyptree ; they are regarded by Gray as a separate 
family, including several genera; Deshayes, on the other hand, proposes 
to unite them; whilst Lamarck commences his observations by saying, 
‘that amongst the conchiferous Gasteropods, no genus is so eminently 
distinct as that of Crepidula, whether as regards the animal or its shell.” 
The fact is, that Lamarck was only acquainted with a few species, the 
typical forms of each division; and, as the intermediate varieties have 
since become known, some authors have been prompted to increase the 
number of genera, whilst others have at the same time advocated the pro- 
priety of diminishing it. For our own part, we see no reason to depart 
from the arrangement of Lamarck ; we refer to the present genus, Crepi- 
dula, that portion of the Capulacea in which the internal appendage 
assumes the shape of a horizontal septum or shelf (the Slipper Limpets), 
and to the following one, Calyptrea, all those in which the septum gradu- 
ally becomes modified, through the trochiform development, to the form 
of a detached cup (the Cup-and-Saucer Limpets). 
The shell of Crepidula may be described as being ovate or oblong, 
transversely elliptic, generally convex at the back, and hollow beneath ; 
the vertex, which in some species almost approaches to the form of a 
spire, is somewhat obliquely turned on one side at the margin, and the 
