﻿UNDULATE. 61 



auteally ; superior border lengthened, straight, or shghtly concave ; lower border slightly 

 curved elliptically ; umbones antero-mesial, elevated, obtuse, and but little recurved. 

 Area narrow, flattened, somewhat raised, with numerous very irregular, rugose, trans- 

 verse plications ; it has a well-defined median furrow, bordering upon which, on each 

 side, is a row of minute tubercles ; the inner and marginal carinae are each represented 

 by a small row of inconspicuous tubercles or knotted terminations of the transverse 

 plications of the area. The escutcheon is depressed and flattened, its length is con- 

 siderable, or nearly equal to the measurement across the valves ; its posteal extremity 

 forms an obtuse angle with that of the area. The costae upon the sides of the valves 

 have but little prominence, the first-formed three or four rows are horizontal or slightly 

 oblique, with small, regular, cord-like tubercles, the succeeding rows form two distinct 

 series ; the anteal series are few, oblique, and very irregularly sub-tuberculated ; for the 

 most part the rows have but little prominence and sometimes become nearly evanescent 

 towards the middle of the valve ; the few last-formed or lower ones are commonly more 

 or less confused or imperfect ; about the middle of the valve this anteal series is replaced 

 by a more numerous posteal series, whose lower extremities form nearly right angles with 

 the other series ; they are regular, narrow, closely arranged, straight, imperfectly tuber- 

 culated, and are somewhat more prominent than the anteal series ; they pass upwards 

 nearly perpendicularly to the marginal carina ; there are about fifteen rows, and their 

 size continues nearly equal even to the posteal extremity of the valve. 



The test is thick, the borders of the valves smooth, and the hinge is remarkable for the 

 great breadth and flattening of the central tooth in the left valve. The larger of our speci- 

 mens has the length, upon the marginal carina, of 3^ inches ; the opposite measurement 

 is 2^ inches ; the diameter through the united valves is inconsiderable. T. produda 

 has occurred only in single valves ; the internal mould is unknown : it is one of the 

 largest and least known of the XJndulatcB ; several young examples have been obtained, 

 these are but little distinguished from similar examples of T. signata. The costte supply 

 the distinguishing features of T. jjroducta, the few widely separated and rather obscure 

 anteal series, and the more closely arranged, but separate, straight, and narrow posteal 

 rows, serve to remove it from T. signata, which has moreover the umbones more mesial and 

 recurved ; the anterior side and border is more produced and rounded ; this arrange- 

 ment of the costse is very distinct from T. V-costata, which has the anteal series much 

 more numerous, and of a different figure, and does not form rows of separate tubercles, 

 It has also some affinity with Lyrodon literatum, Goldf., in the characters of the costas ; 

 but the latter species has the general figure more lengthened and oblong, and the posteal 

 portion of the area has much greater breadth ; the marginal carina also has a row of 

 large rounded tubercles ; the umbones are smaller and less elevated ; the anterior side is 

 also more produced. In T. producfa the whole of the ornamentation has but little 

 prominence, and in some examples it is partially obscured by the plications of growth, 

 which become large and rugose over the lower portion of the valve. 



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