NOTES ON CRUSTACEA FROM FAIRLIE AND HUNTERSTON. 353 



Thalestris forficulus, Claus. — This is a very distinct species of 

 Thalestris. The branches of the first thoracic feet are long and 

 slender in comparison with those of most of the other members 

 of the genus ; it is also a small species. A few years ago Ave 

 described, under the name of Thalestris forficuloides, a Copepod 

 that seemed to be different from Claus's species, but both my son 

 and I are now inclined to regard this as merely a local form of T. 

 forficulus. 



The two apparently undescribed Copepods found in the Hun- 

 terston pools are both very slender and both appear to be rare. 

 Only four specimens of the larger and rarer of the two have 

 been observed, but specimens of the smaller form were somewhat 

 more frequent. These smaller specimens were difficult to notice, 

 because they so closely resembled the bits of fibre with which 

 they were mixed up ; it was on this account easy to miss them, 

 and therefore they may have been less rare than they appeared 

 to be. 



Another Copepod, which I have recorded as Delavalia gies- 

 brechti, var., resembles that species very closely. The typical 

 D. gieshrechti was discovered in Ayr Bay about two years ago. 

 It has a peculiarly broad terminal seta on each of the caudal 

 furca, and it may be distinguished by this character alone. In 

 the Hunterston variety the caudal setae are of the usual form. 

 The arrangement of the setse on the fifth thoracic feet also differs 

 slightly from the typical form, but it agrees in so many other 

 points, that it seems better, for the present at least, to consider it 

 as a local variety of D. gieshrechti than to make a new species of it. 



Among other doubtful Copepods is an Harpactid that partakes 

 somewhat of the characters of Canthocamptus cuspidatus and 

 Moraria poppei, but it differs from them in various ways, but 

 especially in the form and armature of the fifth thoracic feet, and 

 I have not yet decided what to make of it. 



These intermediate forms are very troublesome to the 

 systematist. Their differences are such that they cannot very 

 well take their place beside species already described, yet their 

 affinities with them are so close that one hardly knows where to 

 place them, or what characters to fix upon in order to discriminate 

 them from others. This is where the chief difficulty in the study 

 of the Copepoda comes in. 



