Dr. E. Woodward — On the Genus Cyclm. 535 



bases), taken in connection with the large, slightly-bivalved carapace, 

 suggest that it is an Entomostracan and probably one of the Phyl- 

 lopoda, with a broad cephalic carapace like that possessed by Apiis 

 and by Daphnia ; with large swimming second antenna like the 

 latter, and possibly with a pair of stalked eyes. Cydus, however, 

 differs from the Cladocera in being flattened dorso-ventrally, and 

 from the lowest Crustacea in not apparently possessing any true 

 jaw-parts — the head, with the labrum and mouth, being bent 

 further back than in the living Entomostraca. These differences 

 may either indicate very lowly characters or very much specialized 

 ones. Two views suggest themselves : — 



(1) That these animals were small, free-swimming Phyllopods, 

 with expanded cephalic shield, swimming second antenna, and 

 biramous limbs, the bases of which served as masticatory organs, 

 no true jaws having yet been developed ; the backward position of 

 the mouth may have been brought about in order to allow as many 

 appendages as possible to serve as jaws, as is seen in Limulus. 

 Or, possibly, the beast could attach itself like a living Daphnia by 

 a cement gland on the dorsal side of the head, in which case it 

 might be an advantage to have the mouth as near the freer end as 

 possible and close to the swimming-legs, which were, by their 

 movements, producing the foot-currents. 



(2) The other view is that these animals were ecto-parasitic 

 Phyllopods, although they had not lost their power of free- 

 movement, yet had become specialized in the form of their body, 

 which is flattened ventrally and only slightly convex above, the 

 whole animal being expanded horizontally, unlike most other 

 Phyllopods. This view might account for the two large round 

 structures seen on the ventral surface (PI. XV. Fig. 8), situated one 

 on either side of the body, and close to the anterior margin of the 

 shell. These might possibly represent a pair of ventral suckers, 

 such as are seen in the modern fish-lice ; these structures, what- 

 ever they may be, are evidently enormously developed, and possess 

 great muscles, which produce prominent modifications of the dorsal 

 shield, where they are attached (PI. XV. Figs. 3-5). The great 

 labrum might possibly represent the suctorial tube of Argulus, under 

 cover of which are concealed the reduced mandibles, etc. 



Some of the specimens show curious oblique scars on the coxae 

 of the legs, which may indicate the points of attachment of spines 

 or setae to enable the parasite to stick to its fishy host. 



The following notes refer to the specimens figured on Plate XV. 



1.— Cyclus Jonesianus, H. Woodward. PI. XV. Figs, la, 16. 

 X 3 times natural size. 

 Cijehts Jonesianus, H. Woodw., 1870. Geol. Mag. Vol VII. pp. 657-558, 



"Woodcut, Figs. 1 and 2. 

 Cyclus Jonesianus, H. Woodw., 1878. Mon. Pal. Soc. vol. xxxii. Fossil Mero- 

 stomata, part v. p. 254, pi. xxxii. figs. 46a, b. 



I was at first inclined to the opinion that the specimen of Cydus 

 lent to me by my friend Dr. John Young of Glasgow (see PI. XV. 

 Figs, la, 16), was a new species ; but after having carefully compared 



