cirri, the large palpi, the notched labrum, and the laterally double teeth of the mandibles;--and on the other hand, in the Lepadidæ, the presence of ovigerous fræna, caudal appendages, bullate labrum, and often prominent olfactory orifices. But if the Lepadidæ be compared in these several respects with the other sub-family, or Chthamalinæ, which cannot possibly be removed out of the family of Balanidæ, many of these differences break down and disappear, in some or all of the species. The Lepadidæ include, as has previously been noticed, a much greater range of difference than the Balanidæ; and this is what might have been expected, for it is the most ancient family, and extinction has done its work, separating genera, which, in accordance to analogy, we may suppose were once more nearly connected by intermediate forms. The Lepadidæ, in one sense, may be taken as the type of their order; for they have undergone less "morphological differentiation;" that is, they differ the least from the last larval stage, and seem to give the most _general_ idea of a Thoracic Cirripede. On the other hand, if we mean, as some authors do, by the word type, that form which, in the group in question, has been most modified, and illustrates every peculiarity of its class in the strongest manner, then we must look to the Balaninæ, and to its typical genus, Balanus, for the most Cirripedial form. In this genus the different portions of the carapace differ most, and subserve to a certain extent different ends, and in minute structure are most complicated; here the cementing apparatus, which offers the main characteristic of the whole sub-class, is most complex; here the several pairs of cirri differ most from each other in structure and action; here the peculiar branchiæ (organs apparently derived from the modification of another organ, itself confined to Cirripedes, viz., the ovigerous fræna) are best developed; here the nervous system is most highly concentrated; and, lastly, here we meet with the largest and most massive species of the whole group. 1. Family BALANIDÆ, (or Sessile Cirripedes). _Cirripedia without a peduncle; scuta and terga furnished with depressor muscles; other valves united immoveably together._ TABLE OF CONTENTS. Page