28 A MONOGRAPH OF THE 



colder climates, but the notices of extra-European species are so scanty that we cannot form 

 any idea of their numerical preponderance, nor can we, for the same reason, point out, as may 

 be done in many groups of animals and plants, any particular centres of distribution where 

 they are more especially congregated. With the exception of a few oceanic wanderers, such 

 as Glaums and Scyllaa, which have been met with by almost every voyager, nearly all the 

 Nudibranchs that have been described by naturalists visiting the southern hemisphere belong 

 to the Boridida ; and these show a much greater variety of forms than are to be found in the 

 same family with us. It may, therefore, be inferred that the members of this family are 

 proportionally more numerous in warm climates than in the more northern and colder 

 regions; but were we to take the number of species at present known as a ratio of their 

 preponderance, we should certainly be very wide of the mark, for it cannot be doubted that a 

 great deal of the apparent deficiency of other genera, in comparison with the Boridida, in 

 foreign countries, arises from the want of proper examination, and from the little attention paid 

 by collectors to the less conspicuous forms. It may therefore be expected that when 

 naturalists visiting foreign countries shall have their attention directed more especially to this 

 beautiful race of marine animals, they will not only collect those conspicuous species which, 

 from their size and colour, obtrude themselves upon the eye, but will search out in their 

 hidden retreats the smaller, and often more interesting because more varied, forms that belong 

 to the other families. We cannot doubt that a rich harvest is yet in store for future voyagers 

 in this department of natural history. 



To show the futility of founding generalisations upon imperfect data, we may mention 

 that in a report on the distribution of this order formerly published, we stated on the 

 authority of existing works, that the Borididm of the Mediterranean greatly exceeded the 

 Eolidida in number, and that the latter were not only few but of small size compared with 

 those of colder climates. The more recent Catalogue of the Mollusca of the Gulph of Genoa, 

 by M. Verany, however, contains a greater number of species belonging to the latter family 

 than to the former, and we find the average size is in fact greater in the Mediterranean 

 Bolides than in those of our own shores. But, making every allowance for errors, there still 

 exists a greater proportion of Eolidida in the seas of northern Europe, so that our former 

 inference that Bolts is a northern type of form is probably correct. A comparison of the 

 British and Mediterranean Nudibranchs shows that not more than five species have been 

 ascertained to be common to both localities. These are Antiojpa cristata, Goniodoris castanea^ 

 Bolycera quadrili?ieata, Idalia eleyans, and Boto coronata ; others that have been described 

 under the same name are certainly distinct, and even those we now mention are not all quite 

 free from doubt. 



The few species that are yet recorded as inhabitants of the seas of North America come 

 much nearer to the British forms. Two of them, Bendronotus Reynoldsii {arborescens) and 

 Bolycera illuminata (Lessonii), appear to be identical, and some others are probably so. It is 

 only on the Atlantic shores of Europe, however, that we find any considerable number of 

 species similar to our own. But here again the want of sufficient data prevents our entering 

 upon any exact analysis. The French coast has been very imperfectly explored, and those of 

 Spain and Portugal are, in this respect, nearly a complete blank. The species found in the 

 north of France, on the borders of the British Channel, are, with one or two exceptions, 



