132 BRITISH PALEOZOIC PHYLLOCARIDA. 



PL XXII, fig. 1, looks as if a bivalved test were kept from closing by the inter- 

 vention of jthe sbaly matrix; but it may have been habitually more or less open 

 or gaping. The ventral margin of Dithyrocaris seems to have stood out free from 

 the valves in some specimens, being, as noticed by others, obliquely striate on the 

 upper surface and longitudinally striate below ; and this free edge is folded in 

 under the margin in some instances, PI. XXVII, fig. 5. In Choenocaris it seems 

 to be limited in width, and to remain at right angles with the valve to form a 

 rabbeting joint with the other valve, PI. XXI, fig. 11, and PI. XXII, fig. 1 d, 

 if closed. 



The chief features to be noticed on the carapace-valves are — 



1. Anterior process or spine. 



2. Posterior process or spine. 



3. The middle or dorsal ridge and its flanges. 



4. The juxta-dorsal ridges. 



5. The middle-lateral (mesolateral) ridges. 



6. The cephalic, gastric, or ocular ridges. 



7. The nuchal ridges. 



8. The anterior and posterior notches or medial indentations. 



9. The antero-dorsal and postero-dorsal notches. 



As far as at present known the allied genera comprise the following forms, 

 and perhaps others to which the various caudal appendages and gastric teeth may 

 have belonged : 



'Overriding — 1. glabra smooth, with slight mesolaterals. 



? 2. ovalis | Granulated ; with stronger meso- 



Overriding — 3. grantjlata ... ) laterals. 



Overriding — 4. testudinea ...with transverse lines 



? 5. Scouleri smooth 



? 6. funiculata ...slightly reticulate 



? 7. insignis sinuato-reticulate 



<x> 



"3 



With a dorsal ridge. 

 ( Dithyrocaris.) 



SJD 



a 



S-* ^— < 

 \ aD Eh 



Without a dorsal ridge ? 11. Belli multilineate and reticulate I- 



Overriding-8. Colei j vvlt hweak juxta-) "harpgranules, 



? 9. orbicularis f j i f reticulation, 



Overriding 10. tricornis ... j "' ) and pits. 



? 11. Belli multilineate and reticulate 



Without a dorsal ridge. ■) (12. tenuistriata multilineate aud reticulate -v With 



Gaping bivalve. > < 13. Toungii smooth J-juxra- 



(Ckomocaris.) ) (14. ? Biciiteriana costulate ' dorsals 



Without a dorsal ridge. S 



Closed bivalve. > 15. striata multicostulate 



( Ca lyp tocaris. ) ) 





In the ' Geological Magazine,' December 2nd, vol. i (1874), p. 109, it was 

 suggested that some of the then known species of Dithi/rocaris showed, by the 

 relatively deep central indentations in their anterior and posterior borders, that 

 the carapaces in this " Group B " may have been more acutely bent down at the 



