. . INTRODUCTION. 3 
above thirty-seven fossil species, with almost the certainty—as we shall presently see—of 
very many more being discovered, this plan seemed to me too inconvenient to be followed. 
There are six recent species which I intend, in a future work, to include under Scalpellum. 
Four of them have been raised by Dr. Leach and Mr. Gray to the rank of genera; two 
other unnamed species have certainly equal, if not stronger, claims to the same rank ; 
so again the six recent species of Pollicipes have similar claims to be divided into three 
genera, thus making nine genera for the twelve recent species of Scalpellum and 
Pollicipes. In the majority of cases it would be eminently difficult to allocate the fossil 
species in these nme genera; nevertheless, taking the characters necessarily used for the 
generic divisions of all the other recent Pedunculated Cirripedes, there can be no doubt 
that the formation of the above nine genera might be justified, that is, if we are allowed 
to neglect mere classificatory utility as an element in the decision, and further, if we are 
invariably bound to make as far as possible all genera of exactly the same value. As 
far as utility im classification is concerned, it appears to me clear that the institution of so 
many genera, until many more species are discovered, is highly disadvantageous: with 
respect to making all genera of exactly equal value, this, though eminently desirable, 
appears to me almost hopeless; I know not how to weigh the value of slight differences 
im different valves; or whether a difference in the maxille or mandibles be the more 
important : anyhow, in this particular case, if we raised the six recent species of Scalpellum 
into six genera, they assuredly would not be distinct to an exactly equal degree. Under 
these circumstances I have followed a middle term, and kept Scalpellum and Pollicipes 
distinct,—genera easy to be recognised in a recent state,—which renders the classification 
of the fossil species, though always difficult and lable to many errors, somewhat easier 
than if both genera were united into one, and much easier than if the above nine genera 
‘were admitted. 
APpTyYcHus. 
Before passing to more general considerations, I must offer a few remarks on the 
genus Aplychus, or Trigonellites, masmuch as quite lately a distinguished naturalist, 
M. D’Orbigny,* has adopted, and with much ingenuity supported, the view that these 
anomalous bodies are Pedunculated Cirripedia. It cannot be denied that the general form 
and lines of growth closely resemble those of the Scuta or lateral inferior valves in Lepas 
or Anatifa: nor can it be denied, from what we know of recent species, that the Terga 
(upper lateral valves) and Carma (dorsal valve), which on M. D’Orbigny’s view must be 
considered as absent, are the most likely valves to disappear from abortion. But there 
are points of difference which, as it appears to me, are of far greater importance than the 
! Cours Elémentaire de Paléontologie, 1849, vol. i, p. 254. 
