32 BRITISH FOSSIL TRIGONIiE. 



succeed are in proportion much more widely separated, very irregular and oblique ; they 

 approach the marginal carina at a right angle. Anteally the costae become attenuated 

 and sub-tuberculated, their direction is more irregular and variable ; not unfrequently they 

 form a kind of undulation, and have the tubercles indistinct or cord-like ; in other 

 instances their direction anteally is nearly straight or horizontal, and invariably there is a 

 supplementary rib formed upon that side. The posteal extremities of the costse never 

 reach to the marginal carina ; it is separated from them by a smooth diagonal space for 

 the lower three fourths of its length, but this space is neither considerable nor altogether 

 uniform upon each valve in all specimens. The number of costae are usually about 

 thirteen, but occasionally sixteen ; the tubercles upon the few later-formed costse are 

 large and obtuse posteally, but their number upon each row and their figure are very 

 variable, some costae having only eight and others about thirteen tubercles. 



This is one of the most elongated and irregular of the Clavellata ; it is the Cornbrash 

 shell attributed to T. clavellata in the lists of Cornbrash fossils given by Phillips, 

 Williamson, and Bean. In irregularity of the costae it quite equals T. irregularis, Seebach, 

 that beautiful Oxford Clay shell so long and well known at Weymouth ; but a comparison 

 of adult forms in the two species will at once show their distinctness. It approaches in 

 figure more nearly to T. Voltzii ; but in commencing our comparison with the umbones 

 we find that in T. Scarburgensis they are less produced and recurved ; the anteal side is 

 more produced, and has much less convexity ; the superior border is much straighter, 

 resulting from the more raised superior border of the escutcheon ; the rows of costae are 

 much more irregular, the tubercles are smaller and less raised ; they do not terminate 

 abruptly anteally, but become gradually attenuated and sub-tuberculated. 



Young specimens having only eight or nine rows of costae have not any strongly 

 defined specific characters : they sometimes have portions of the granulated tegument 

 preserved. The left valve is not unfrequently found with its ornamentation imperfectly 

 developed, as in our specimen Plate IV, figs. 2, 3. The latter figure, which has been 

 exceeded in its irregularity, appears to have resulted from an atrophized condition of the 

 mantle upon that side ; a defect which is equally conspicuous upon the left valve of the 

 young specimen, fig. 4 upon the same plate, and is not, therefore, a concomitant of 

 advanced growth. 



Slratigrap/tical position and localities. It is not uncommon in the Cornbrash upon 

 the northern side of Scarborough Castle Hill and in Cayton Bay. The late Dr. Porter 

 obtained it in the same formation near to Peterborough. The officers of the National 

 Geological Survey state that it is an abundant fossil at several localities in the South Lin- 

 colnshire district. 



