﻿COSTAM. 



167 



figures of the bounding carinas, the considerable angle which the surface of the area 

 forms with the other portion of the valve, together with the more lengthened costae, and 

 the absence of truncation anteally, serve in the aggregate clearly to separate it from 

 T. costata, and also from others of the same section. 



For comparison with T. Meriani the reader is referred to that species. 



The T. reticulata of Agassiz (' Trig./ table 2, fig. 10), which is exemplified by a 

 single fragment, may represent a large and compressed example of T. monilifera ; the 

 apparent absence of a median carina in this instance is similar to a like variation in 

 occasional Dorsetshire examples, in which the median carina divides in fully developed 

 forms into two or three costellae, and the carina thus disappears. 



Without hesitation the T.papillata of the same author, 'Trig.,' tab. 5, figs. 10 — 14, 

 may be referred to T. monilifera ; here again the median carina divides into costellae ; the 

 prominence of these and of the bounding carinae may be taken to represent a small 

 Dorsetshire example of our species. 



I would exclude T. monilifera, Quenst. ' Handbuche der Petrefacten-kunde,' tab. 43, 

 fig. 15, which represents a small Trigonia having very numerous small rows of costae and 

 also a species, Quenst., Jura, tab. 93, fig. 4, bearing the name T. costata-silicea ; the 

 latter may possibly agree with a delicately ribbed species of the Upper Oolites, T. supra- 

 jurentis, Ag., Trigonies, tab. 5, figs. 1 — 6, page 42. The latter is unknown in the 

 Kimmerictgian strata of Britain. 



Positions and Localities. The lower beds of Kimmeridge Clay in the vicinity of 

 Weymouth, of Wotton Basset, and of Swindon, have produced examples, some of which 

 in their general dimensions much exceed any other species of the costatae ; these, 

 however, are usually more or less compressed or distorted ; it also occurs in the Coralline 

 Oolite of Wilts and of Weymouth ; at the latter locality specimens deprived of the test, 

 and ill-preserved, have recently been obtained in a red pisolitic iron rock at Abbotsbury, 

 and forwarded to me by Mr. J. T. Walker, of York. 



Foreign examples recorded by Agassiz and by D'Orbigny have been obtained in the 

 Terrain a Chailles or Lower Calcareous Grit at Argan (Haut Rhin), Birze Environs of 

 Bale. Besancon (Doubs), Neuvizi, Trouville, Nantua, Marans. Also by Grewingk, at 

 Poplinacny, in the Province of Kowno ; Lithuania, there known as T. costata. 



Trigonia Meriani, Ag. Plate XXXIII, figs. 1, 2, 3. 



Trigonia costata, Young and Bird. Survey of the Yorkshire Coast, pi. viii, fig. 19, 



p. 225, 2nd edit., 1828. 



— — Phillips. Geol. of Yorkshire, p. 228, 1st edit., 1829. 



— Meriani, Agassiz. Trigonies, p. 41, tab. xi, fig. 9, 1840. 



