CLAVELLATA. 27 



downwards to the lower border at right angles to the carina ; they are straight or 

 occasionally waved. The other specific features may be conveniently given under two 

 separate descriptions of individual specimens, the originals of our figures ; of these the 

 Yorkshire shell appears to be of more advanced growth than the other. 



From Lower Calcareous Grit of Cuninor, From Lower Calcareous Grit of Filey Point, 



Oxfordshire. Yorkshire. 



The median and inner carinas are represented each The median and inner carinas are slightly elevated ; 



by a row of regular rounded tubercles the marginal each consists of an irregular series of unequal trans- 

 carina is distinctly elevated, and for two thirds of its verse varices, which are continuations of the large 

 length has a row of well separated rounded tuber- irregular plications that cross the area ; the marginal 

 cles ; towards the posteal extremity these become carina is elevated, consisting of squamous varices or 

 large plications. The costse consist of fifteen rows large plications, which are continued across the area, 

 of large and moderately elevated rounded tubercles, The costse consist of about fourteen or fifteen rows 

 which, towards the border, become continuous rope- of oblique, but straight or somewhat waved, broad, 

 like varices ; the last three rows are altogether con- depressed ridges, each of which has about thirteen 

 tinuous and rope-like ; the lines of growth are dis- narrow, oblong or slightly rounded varices, which 

 tinct upon the lower portion of the valve, and are are much impressed by the large, irregular, longitu- 

 more conspicuous where they cross the area. dinal plications upon the sides of the valves ; the 



varices are compressed obliquely from the direction 

 of the carina, and therefore not in the direction of 

 the lines of growth. 



B' 



Apparently these specimens exhibit the extremes of variability to which the species is 

 liable in the surface-ornaments, and also in the figure ; the Cuninor shell is unusually 

 convex, and is also very short compared with the height; the other has the length 

 greater than usual in proportion to the height. 



From T. clavellata, and not less so from other of the Clavellata, it is readily distin- 

 guished by the sub-trigonal depressed figure, and large, nearly perpendicular, nodulous 

 varices upon the sides of the valves. It has some resemblance to T. Suevica, Quenst., but 

 is much shorter, and the apex more elevated, with fewer oblique varices. 



Large blocks of stone, detached by marine action from adjacent beds of Lower 

 Calcareous Grit, at Filey Point and at the Castle of Scarborough, contain rough and usually 

 ill-preserved specimens of this Trigonia, of a size comparable to the largest known 

 examples of the genus. In common with other insufficiently known clavellated forms it 

 has been assigned to T. clavellata, Sow. Upon the coast of Yorkshire the valves are 

 seldom found disunited ; they are in contact, or spread open ; originally held together by 

 the ligament, or in the worst-preserved specimens the calcareous spar into which the 

 test was transmuted has disappeared, and the rough, brown, grit-stones still show the 

 ornamentation of the valves more or less imperfectly. The foregoing remarks are founded 

 upon eight examples of various dimensions, two of which are from the same formation at 

 Cumnor, Oxfordshire ; three have the valves in contact, the others are in a less satisfactory 



