CYTHERURA. 



201 



19. Cytherura clathrata, G. 0. Sars. Plate XI, figs. 1 — 4. 



1865. Cytheruka clathrata, G. 0. Sars. Overs. Norg. mar. Ostrac, p. 77. 

 1868. — — Brady. Monog. Rec. Brit. Ostrac, p. 446, pi. xxix, 



figs. 43—46. 



Carapace, as seen from the side, triangular, highest in the middle^ greatest height 

 almost equal to three fourths of the length ; anterior extremity narrowed, subangular, 

 divided into three or four short, broad teeth ; posterior also narrow, produced into an 

 almost obsolete rounded beak ; superior margin much elevated in the middle, sloping very 

 steeply and with a scarcely perceptible curve to each extremity, inferior distinctly convex, 

 more or less sinuous. Outline, seen from above, ovate, widest in the middle, greatest 

 width equal to more than half the length, extremities mucronate. End-view triangular, 

 with arcuate sides and flattened base ; height much greater than the width. Each valve 

 is traversed by one conspicuous central longitudinal rib, from which several other 

 more or less distinct curved ridges diverge laterally, the interspaces excavated into 

 numerous large fossae ; ventral surface longitudinally striated. 

 Length, ^th of an inch. 



This is the largest of the genus, and in general appearance bears considerable 

 resemblance to the preceding species. In the Post-tertiary formations it is apparently 

 of much more frequent occurrence than C. ceUuIosa, though comparatively rare in our 

 seas at the present day. 



Distribution. Becent. — Baffin's Bay, Norway, Great Britain, Ireland. 



Fossil. — Scotland : Tangyburn, Duntroon, Crinan, Loch Gilp, Blackburn, Cumbrae, 

 Dumbarton, Cartsdyke, Paisley, Dalmuir, Old Mains, Inch Lonaig, Drip Bridge. 

 Ireland : Portrush. England : Bridlington. Norway. 



Genus 9 — Cythero^teron, G. 0. Sars. 



Valves mostly subrhomboidal, tumid, unequal, and different in shape, the right 

 valve more or less overlapping the left on the dorsal margin ; surface of the shell 

 variously sculptured, punctate, papillose, reticulated, or transversely rugose, ventral 

 surface produced laterally into a prominent rounded or spinous ala ; posterior 

 margin produced into a more or less distinct but obtuse beak. Hinge formed by 

 two small terminal teeth on the right and by a minutely crenated median bar on 



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