UNDULATE. 63 



angle, and the last-formed three or four rows pass perpendicularly down to the lower 

 border ; their anteal or lower extremities are for the most part united to the extremities 

 of the more numerous anteal series, with which they form a considerable undulation or 

 angle, which is always less than a right angle ; not unfrequently, however, the few 

 last-formed anteal costse are altogether irregular, presenting only small confused 

 tubercles. 



This appears to be the shell indicated by the author of the ' Geology of Yorkshire,' 

 who gave a short notice of it at page 156 of the first edition of that work, but without 

 any figure. It is nearly allied to T. angulaia, Sow., in the general arrangement of its 

 ornamentation ; but it differs from that species constantly and materially in the general 

 figure, which is much more broad and expanded posteally, — so different from the narrow, 

 concave, and delicately plicated area of T. angulata ; it is also without the undulation 

 upon the lower borders of the latter species, and its rows of anteal costae are more 

 numerous ; but usually the outline of the two forms will at once show their distinctness. 

 It is also somewhat allied to T. Moretoni, M. and L., but is more oblong, with a much 

 wider and more rugose area ; the marginal carina is much smaller, the costse are also 

 more disunited mesially, and have the tubercles smaller, and the costae less ridge-like. 



The two small lower figures of plate Ixxxvii ' Mineral Conchology,' represented as a 

 variety of T. clavellata, have some affinities with our species in the general figure, and the 

 characters of the costae ; but the area is destitute of the large transverse rugose costellse i 

 there are also indications of a small median carina, it is probably therefore distinct. 



T. comjjta, Lye, of the Collyweston Slate, is also nearly allied to it, both in the general 

 outline and the ornamentation. The flattened condition of the slate species prevents any 

 comparison of the convexity, but the costae in adult specimens are fewer ; the area is 

 smooth, and has three distinct tuberculated carinas, — characters which are so different from 

 that portion of T. covjungens as to compel their separation as species. 



Sir aiigrcqjJiical position and Localities. During many years T. conjmigens remained one 

 of the more obscure and doubtful forms of Trigonia, and was usually omitted, even in the 

 lists of Yorkshire fossils ; this arose from the very few words of description allotted to it 

 by Professor Phillips in his ' Geology of Yorkshire,' and not less so to the very intract- 

 able stone of the Millepore-bed, a hard, rough, semi-ferruginous stratum at Cloughton 

 Clifis, to the northward of Scarborough. A considerable number of examples of T. 

 conjungens have been obtained from that locality, but only a few have been separated well 

 preserved. Brandsby, Yorkshire, was the locality given by Professor Phillips, at which 

 place the beds are not now accessible : it has also been obtained in the White Oolite 

 of Whit well, in the same county. The Cloughton specimens are associated with 2\ 

 recticosta, and a considerable number of Inferior Oolite Conchifera. It also occurs in the 

 same stratum exposed at Cayton Bay, to the southward of Scarborough. Specimens are 

 in the Museum of the Yorkshire Philosophical Society at York, also in the collection of 

 Mr. Reed, of the same place ; in the collection of Mr. Leckenby, now forming part of 



