28 B1UTISH FOSSILS. 



case might be made out for the relegation of the Pterygoti to the 

 latter order. (See p. 36, note.) 



The evidence which has led me to this conclusion can only be 

 appreciated when the structure of the Copepoda is fully understood. 

 I will, therefore, proceed to give a- brief account of that structure, 

 and I am the more willingly led to do this as I have found it 

 difficult, notwithstanding the valuable labours of Milne Edwards, 

 Baird, Dana, and others, to arrive at a clear and connected view of 

 the anatomical characters of this difficult group. 



The genus Calanus, containing a large number of marine species, 

 is perhaps best fitted to furnish a typical illustration of copepod 

 anatomy. The species represented in Plate XVI. fig. 1 , was taken in 

 the North Atlantic. It is about one-eighth of an inch long, and (after 

 preservation in spirit) has a lightish brown colour. The body pre- 

 sents a large carapace anteriorly, succeeded by a number of thoracic 

 and abdominal segments. The cephalo-thorax has the form of a 

 long oval, truncated and slightly excavated behind, the postero- 

 lateral angles of its last segment being but very slightly produced. 

 The abdomen has not a third the length of the head and thorax, 

 and is terminated by two elongated lobes provided at their extre- 

 mities with long setse. The antennules are as long as the body. 

 The cephalo-thorax consists of six segments, the anterior of which is 

 about equal to the three following ones in length, and forms a cara- 

 pace. The abdomen presents four obvious segments, but the first 

 appears to consist of two somites united together, and it is probable 

 that the two terminal styliform lobes may represent a somite, in 

 which case there would be six somites in the abdomen. The anus 

 opens between the bases of the terminal processes. 



The carapace is not prolonged, either along its posterior margin or 

 laterally, but merely overlaps the tergum of the succeeding somite 

 in the ordinary manner. Anteriorly, it extends beyond the bases 

 of the antennules, and is produced forwards and downwards, be- 

 tween them, into two delicate pointed processes, slightly constricted 

 at their bases, which are about equal in length to the two basal 

 joints of the antennules. The latter are slender, of even thickness, 

 and divided into twenty-four setose joints. 



An endophragm, or inflexion of the chitinous integument, dis- 

 tinctly separates the sternum of the antennulary, from that of the 

 antennary, somite. Viewed laterally, the former appears concave, its 

 posterior portion sloping backwards and downwards at an angle 

 with the anterior. The anterior half of the antennary sternum, on 



