BIVALVIA. 91 



hinge with an obtuse cardinal tooth in each valve, which is received into a corres- 

 ponding cavity in the other valve; occasionally in the right valve there is a small 

 anterior, and in the left a small posterior accessary tooth or prominence upon the 

 margin of the cavity ; lateral teeth are large, posterior, and approximate in each valve, that 

 of the left valve projecting and received into a depression of the tooth or callosity of the 

 other valve. Muscular impressions oval ; pallial impressions simple, faintly marked. There 

 is no lunule ; the margin of the right valve anterior to the umbo forms a thickened projecting 

 fold, which covers the tooth of the other valve, and is received into a corresponding receding 

 portion of the margin of that valve ; so that the junction of the valves anterior to the umbo 

 has a sinuous flexure. 



In the typical species, T. donaciformis, which is an Inferior Oolite shell, the lateral teeth 

 are remarkably large; and they are nearly equally conspicuous in the Hettangia 

 Deshayesea, Terquem, and H. Broliensis, from the Lias of the Moselle and the Meuse, figured 

 by M. Buvignier ; but the other Liassic species described by that author, coincide in their 

 dental characters more nearly with our Great Oolite species of this genus. In these, the shells 

 are more delicate, the hinges are smaller and more elongated, the teeth are less projecting, 

 and the cardinal tooth of the left valve is elongated forwards, somewhat upon the anterior 

 border ; the lateral teeth are variable in their prominence, and not uncommonly the tooth 

 of the right valve is indistinct or obsolete. When the valves are much flattened, the posterior 

 aperture becomes narrow or not distinguishable. The figure of Tancredia varies according 

 as the anterior or posterior sides are the most produced ; but more commonly the posterior 

 side is the shorter one, and when it is much truncated, the figure then nearly resembles that 

 of the recent Donaces. All the species at present known are destitute of ornament ; they are 

 smooth, and exhibit but indistinctly the lines of growth. The margins of the valves are 

 smooth, and, independently of the posterior aperture, there is a general irregularity in the form 

 of the margins, so that they are not close fitting along their extent. In England, Tancredia 

 has only hitherto been noticed in the lower Oolitic rocks. M. Buvignier and M. Terquem 

 have recognised eleven species in the Lias of France, and Dr. Dunker one from Halber- 

 stadt. To the geologist a knowledge of this form is of importance, as the species appear 

 to be very limited in their vertical range, and hitherto it has not been discovered that any 

 one of them is common to two formations. The profuseness with which T. brevis is 

 distributed in the shelly beds of the Minchinhampton Great Oolite, and the young of 

 T. donaciformis in the shelly freestone of the Leckhampton Inferior Oolite, is such, that 

 each becomes the most abundant bivalve of their respective localities ; the valves are always 

 disunited, and casts are unknown. 



In looking to the affinities of this genus, we discover a near approximation almost an 

 actual passage into a group of Oolitic forms, which are as yet very imperfectly known, 

 and of which Corbis Items, Sow., and Corbis deprcssa, Buvig., are examples. Three 

 other species have been obtained from the Inferior Oolite of the Cotteswolds, and one from 

 the Coralline Oolite of Malton. In all of these a smooth surface is coincident with a 



