ADDENDA. 207 



always very sparingly, and apparently is not limited to a single bed or horizon of that 

 formation. Collected during the last half century by geologists and local observers it 

 remains a somewhat rare species, and is absent in collections of Inferior Oolite fossils 

 which are unconnected with the Cotteswolds. 



Distinctive Differences. — T. j»(X?^cico5?'a is the smaller of the two species; its general 

 convexity is greater, the anterior side is shorter, giving to the umbones a more anteal 

 position ; the marginal carina in its upper portion has a row of well separated and 

 rounded tubercles ; these do not occur in T. angulata. The rows of costas have much 

 variability in both species, but more especially in T. paucicosta ; usually these terminate 

 posteally with two or three large nodes in each row, or these are sometimes united and 

 become a single varix. 



In T. angulata the posteal portions of the rows of costse have much greater uniformity ; 

 in common with the Undulata generally they are subtuberculated, become attenuated, 

 and curve upwards to the carina with a graceful undulation (see PL XIV, fig. 6 ; 

 PI. XXXVII, figs. 7, 8, 9). These differences indicate the propriety of a zoological not 

 less than of a stratigraphical separation. 



As a correction to p. 59, line 4, read, " few examples of Trigonia paucicosta have 

 occurred at that locality." 



Trigonia ingens. Lye. Plate VIII, figs. 1, 2, 3 ; Plate XXXVI, figs. 5, 6, p. 24. 



Trigonia ingens of the Middle Neocomian formation, compared with T. signata of the 

 Inferior Oolite, Zieten's variety. 



Subsequent to the publication of the figures and descriptions of Trigonia ingens, 

 numerous fine examples, with the test preserved and representing every stage of growth, 

 have been obtained by Mr. Keeping, of the Woodwardian Museum, Cambridge, in the 

 Middle Neocomian formation at Acre House, near Tealby ; the bed is a brown ferru- 

 ginous pisolite ; a portion of the rock worked for iron-ore at that locality is described by 

 Professor J. W. Judd, * Quart. Journ. Geo!. Soc.,' vol. xxiii, p. 227. Two additional 

 figures of small specimens from that locality wnll be found, Plate XXXVI, figs. 5, 6 ; the 

 general aspect is altogether that of the Jurassic Clavellatce, and bears so considerable a 

 resemblance to British specimens of 2. signata from the Inferior Oolite that without care 

 it might be mistaken for that species. 



Compared with the Jurassic shell the general figure has much greater convexity, or 

 is more ovately oblong ; the area is more narrow, steep, convex, and less expanded ; its 

 transverse plications are more prominent, rugose, and irregular; its bounding carinse are 

 less distinct, and sometimes disappear, or degenerate into plications ; the position of the 

 median carina is occupied by a groove ; the umbones are smaller, more pointed, and 

 anterior ; the rows of costse have less curvature, they are more nearly transverse or 



