UNDULATiE. 83 



become simply transverse and lose the falciform aspect. The materials upon which the 

 foregoing description is founded are all more or less imperfect, but in the aggregate they 

 exemplify nearly the whole of the more important features. The specimen with the 

 ornamentation preserved over the greater part of the costated surface is in two portions, 

 exhibiting only a moiety of the marginal carina, and is destitute of the area. The 

 internal mould gives the general outline and proportions ; the large striated hinge teeth, 

 the muscular scars, and pallial lines ; fortunately also, owing to the thinness of the test, 

 the exterior ornamentation is obscurely visible, and even the posteal portion of 

 the area has delicate, faintlv-marked, transverse striations. The mould and also the 

 specimen belonging to Mr. Grant have the few last-formed posteal portions of the costae 

 almost effaced, a condition which appears to be a concomitant of the last stage of growth. 

 The specimen figured with the surface preserved retains its costae at that portion of the 

 valve. 



The third specimen, which exhibits the greater portion of the area, is slightly defective 

 near to the pallial border, and also at the apex ; it is a gutta-percha pressing taken from 

 a well-preserved external cast, kindly forwarded by the Rev. Mr. Joass ; the compact 

 siliceous rock has retained the impression of the surface of the shell with minuteness and 

 delicacy. In the aggregate the examples figured upon our Plate appear sufficiently to 

 elucidate the species. 



Affinities. No one of the British Undulata presents any near approximation to T. Joassi ; 

 there is, however, one foreign Trigonia from the Oxford Clay of Gundershofen, which 

 might possibly be mistaken for it, the Lyrodon litteratum of Goldfuss (' Petref.,' vol. ii, 

 p. 200, tab. 136, fig. 5 h) ; the largest figure is the nearest ally ; excluding other testacea 

 upon the same plate, also named litteratum, which belong to two other species ; the 

 broken specimen pertains to the Inferior Oolite, the other to the Neocomian formation. 

 Limiting the litteratum of Goldfuss in this manner, it will be found to have the area 

 larger than in our species, which is also without the row of distinct rounded tubercles 

 upon the marginal carina ; upon the other portion of the valve the species of Gunders- 

 hofen possesses in its ornamentation a certain amount of resemblance to T. Joassi, 

 differing from the latter in the posteal portions of the costse, which are larger and for the 

 most part almost perpendicular and disunited from the other portions ; the anteal costae 

 are also larger and less numerous. For examples of this fine species of the Widulatcs I 

 am indebted to the kindness of Mr. J. W. Judd, who obtained them at Brora, in the far 

 north of Scotland, during long and persevering researches in the Jurassic rocks of that 

 little-known region. The imperfect specimen in two portions, but having the surface 

 preserved, is from plaster casts of originals in the British Museum, obtained by Mr. 

 Charles Peach in the Brora region. The species is dedicated to the Rev. J. M. Joass, of 

 Golspie, at the suggestion of Mr. Judd, as a fitting recognition of his own obligations to 

 that gentleman, for untiring efforts in his assistance, during the survey of a region 

 heretofore but little explored by geologists. 



