CLAVELLAT^E. 39 



Trigonia irregularis, Seed. Plate V, figs. \, a, b, 2 ; PL VII, fig. G. 



Trigonia, Damon. Geol. Weymouth, Suppl, 1860, pi. ii, fig. 3. 



— Seebach. Der Hannoversche Jura. See his obs. on T. triguetra. 



Shell ovately trigonal or oblong; umbones antero-mesial, prominent, and recurved; 

 anterior side short, moderately convex, slightly truncated ; its lower portion curved with 

 the lengthened lower border, which is slightly sinuated near to its posteal extremity ; the 

 posterior or superior side has its border concave and terminally rostrated. 



The escutcheon is very large and depressed ; its length exceeds the half of the entire 

 length of the shell; its superior border is only slightly raised. The area is narrow, 

 having three tuberculated carinas ; the inner and median carinas have each a row of small, 

 transverse, nodose varices rather distantly arranged ; these are ultimately lost in the large 

 posteal plications of growth ; the marginal carina is small, consisting, for about a third of 

 its length, of a narrow, elevated, finely-indented ridge ; subsequently it acquires small 

 transverse nodose varices similar to those of the median and inner carinae. The transverse 

 plications upon the area are for the most part, small excepting near to the apex, where its 

 surface is occupied by about eight regular, narrow, transverse costellae. The superior 

 half of the area is the more depressed, and has sometimes a minute line of tubercles 

 bordering upon the mesial furrow, and parallel to the median carina. The other portion 

 of the valve has about fourteen rows of slightly elevated costae decked with distinct, 

 elevated, conical, pointed, and unequal tubercles ; the first-formed six or seven rows are 

 regular and concentric ; those which succeed are more or less irregular both in their 

 direction and in the size and arrangement of the tubercles ; the anteal portions of the 

 rows become broken and confused ; the tubercles adjacent to the border are the smaller, 

 and are often compressed laterally. The posteal extremities of the rows are separated 

 from the marginal carina by a smooth and slightly-depressed space which widens 

 downwards, and terminates at the lower border in a well-marked undulation of the border ; 

 the number of tubercles in the rows varies from eight to ten. The figure in Mr. Damon's 

 ' Supplement ' is an extreme example of that general irregularity in the arrangement of 

 the tubercles which Seebach has adopted as a name for the species. The large imperfect 

 specimen which we have figured (Plate V, fig. 2) exhibits the greatest irregularity 

 observed ; the small specimen (Plate VII, fig. 6) is an example of the forms in which the 

 irregularity in the rows of tubercles is so slight as to be quite inconspicuous. The 

 smooth ante-carinal space is always present but varies in its size. 



Stratigraphical positions and localities. In the Oxford Clay of Weymouth it is 

 moderately abundant. It has occurred, also, less frequently in the Kimmeridge Clay of 

 Wootton Basset, Wilts. 



