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 PoUicipes have similar claims to be divided into three 

 genera, thus making nine genera for the twelve recent species of Scalpellum and 

 PoUicipes. In the majority of cases it would be eminently difficult to allocate the fossil 

 species in these nine 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 in classification is concerned, it appears to me clear that the institution of so 

 many genera, until many more species are discovered, is highly disadvantageous : vnth. 

 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 

 in different valves ; or whether a difference in the maxillae 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 PoUicipes 

 distinct, — genera easy to be recognised in a recent state, — which renders the classification 

 of the fossil species, though always difficult and liable to many errors, somewhat easier 

 than if both genera were united into one, and much easier than if the above nine genera 

 were admitted. 



Aptychus. 



Before passing to more general considerations, I must offer a few remarks on the 

 genus Aptychus, or Trigonellites, inasmuch as quite lately a distinguished natm'alist, 

 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 Carina (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 Elementaire de Paleontologie, 1849, vol. i, p. 254. 



