534 Dr. H. Woodward — On the Genus Cyclus. 



One specimen, however (discovered amongst those obtained by 

 Mr. Stock from Eskdale), besides exhibiting a number of fragments 

 of appendages, scattered irregularly over the surface of the matrix, 

 shows one of the middle limbs in position, still attached to a shield 



Fig. 2. — Cyclus testudo, Peach ; Langholm. Enlarged 4 times natural size. 



a. One of the hiramous swimming -feet which remains attached in situ.^ 



b. Base of another appendage, broken off close to the edge of the shield. 



of Cyclus testiido. This limb is a typical biramous appendage, such 

 as we find in the lower Crustacea, and persisting as the swimmerets in 

 the Decapoda; that type of limb, in fact, which is generally believed 

 to have been the most primitive form, and which is only a slight 

 modification of the typical Phyllopod limb. The numerous broken 

 fragments of appendages scattered over the surface of the matrix 

 appear to have been of the same kind. We may, therefore, conclude 

 it is most probable that the oral and post-oral appendages of Cyclus 

 were all of the same type, i.e. simple swimming-legs. 



The V-shaped median plate between the bases of the appendages 

 must either be the coalesced sternites, or an enormously developed 

 labrum. Against the first view we have the fact that this structure 

 is apparently continued without a break up to the anterior margin 

 of the ventral surface, thus giving no place for the mouth. 



If, as the presence of the large antennae and the swimming-legs 

 prove, Cyclus is a Crustacean, then we should naturally expect 

 to find a labrum ; and as there is no other median-lobe-like 

 development of the ventral integument, we must, I think, conclude 

 that the structure represents an enormously developed labrum, and 

 that the mouth opened at the posterior extremity of that structure 

 — an exaggeration of what is met with in the Cladocera, where the 

 mouth opens well back in the shell under cover of a great labrum. 

 This would be further strengthened by the convergence of the 

 bases of the limbs towards its posterior border, a condition similar 

 to that met with in Li7nulus (and supposed by Bernard to have been 

 the primitive condition of the jaw-parts of all Crustacea), these 

 bases serving probably, as in Limulus, as masticatory organs. 



Affinities. — The presence of antennse and biramous swimming- 

 legs prove undoubtedly that Cyclus was a Crustacean. The large 

 size of the former and the homogeneous nature of the rest of the 

 appendages (all biramous swimming-legs,' with possibly masticatory 



^ The branching of the two rami may be due to rod-like deposits of lime in a 

 paddle -shaped fin, two such structures being attached to each limb, representing the 

 endopodite and exopodite. A slight indication of this is visible in the specimen. 



