St. Andrews Marine Laboratory. 471 



raent again ia confined to the ventral edge instead of passing 

 along both dorsal and ventral edges as well as some distance 

 up the sides, as in the Gadoids. 



Except in the tail the young Gadoid of the same size has 

 only embryonic rays in the continuous marginal fin, while in 

 Coitus a considerable number of rudimentary true rays occur 

 both dorsally and ventral ly (10 or 11 dorsally and 6 ven- 

 trally). In the ventral the true rays commence behind the 

 anus. Those in the dorsal begin just above the latter, that is 

 in the posterior division of the body. No permanent rays 

 appear in the dorsal and ventral marginal fin, even though 

 the example exceeds the Cottoid of this stage in length. 



A very evident difference occurs in regard to the tail in 

 those of equal length. Thus, the hypural and epiural ele- 

 ments are more or less equally developed dorsally and ven- 

 trally in the Gadoids, the ventral series, however, terminating 

 in one or two larger cartilages. The tapering notochord is 

 straight and extends considerably beyond both series. True 

 caudal fin-rays, moreover, are developing both dorsally and 

 ventrally — giving the tail a peculiarly symmetrical or 

 " feathered " appearance. 



On the other hand, the notochord in Coitus is somewhat 

 less finely tapered, it has a thicker sheath, and the hypural 

 elements alone are conspicuous in the form of a large inferior 

 and two upper cartilages. The permanent caudal rays are 

 developed only interiorly, while the whole dorsal half and the 

 region extending to the last ray of the dorsal fin have 

 embryonic rays. 



5. On the A^jpearance and Disappearance o/" Lucernaria 

 and other Forms. 



About thirty years ago shannies were extremely common 

 in the rock-pools at St. Andrews and under moist seaweeds 

 on ledges and elsewhere amongst the rocks. Now it is diffi- 

 cult to secure a few adults over a wide area of the same region. 

 One of the most abundant Crustaceans on the beach at all 

 seasons of the same period was the little Portumnus variega- 

 tus, dried specimens occurring here and there daily on the 

 sands. This species is now one of the rarest of the group, 

 no specimen having been procured for a long time. On the 

 other hand, Portunus holsatus has apparently increased in 

 numbers. Small examples of Lucernaria formerly occurred 

 on seaweeds, especially Fucus serratus, both at the rocks 

 near the mouth of the harbour (where they were first found 

 by Miss Ott^ and Prof. G. E. Day) and in the extensive 



