CLAVELLATA. 23 



surfaces have frequently been worn by attrition, but there is never any appearance of 

 compression or distortion ; many valves are broken, so that only a minority have the 

 surface ornaments well preserved. 1 



Trigonia Bronnii, Ag. Plate IV, fig. 8. 



Ltrodon clatellatum, Bronn. Lethsea Geognostica, 1834 — 1838, pi. xx, fig. 3. 

 Trigonia Bronnii, Agassiz. Trigonees, 1840, p. 18, pi. v, fig. 19. 



— — D'Orbigny. Prodr. de Paleont, ii, 1850, p. 16, No. 259. 



— — Hebert. Jour, de Conchyl., 1861, pi. vii, figs. 4, 6, et pi. viii, 



figs. 1, 2, 3; Note sur les Trigonies clavellees de 

 l'Oxford Clay et du Coral Rag. 



Compared with T. clavellata, Sow., and T. perlata, Ag., the chief distinguishing 

 feature consists in the straightness or horizontal directions of the rows of costae which 

 approach the marginal carina nearly at right angles, and the costae last formed take 

 nearly the direction of the lower border. The irregularity and inequality in the tubercles 

 is also remarkable ; usually the second and third tubercles from the carina are larger than 

 the others, and the last-formed one or two rows are smaller, irregularly knotted, and 

 cord-like ; their general direction is nearly horizontal, so that all the rows have their 

 extremities upon the anterior border. Near to the apices the rows are nearly plain, or 

 only slightly crenulated. The escutcheon nearly resembles that of T. clavellata, excepting 

 that it is somewhat larger, and has also greater length ; the area and its ornamentation 

 do not offer anything remarkable, its posteal truncation is usually greater than in our 

 figure. The lines of growth are less conspicuous than in the allied species. Measure- 

 ments of the dimensions are of little utility in a species whose figure varies considerably. 

 More commonly the tubercles upon the costae are fewer and larger, and sometimes more 

 scabrous, than in either T. clavellata or T. perlata. The usual figure of the shell is also 

 less elevated or more oblong ; in a multitude of examples which I have examined from 

 the Coral Rag of Glos these distinguishing features are persistent. 



Stratigraphical position and localities. In the vicinity of Weymouth it has occurred 



1 The following section in descending order shows the position of the Trigonia bed at Pickering in 

 the Coralline Oolite : 



1. Rubbly coarse limestone with large coralline masses. 



2. Thick bed of oolitic building-stone. 



3. Hard band, one foot thick, full of shelly fragments, and of disunited valves of Trigonia perlata. 



4. Tbick bed of oolitic building-stone. 



5. Flaggy, thin-bedded, hard oolite, full of small mollusca, Cerithium and Nerinaea (basement bed of 



the Coral Rag.) 



6. Yellow, hard, subsiliceous sandstones of the Lower Calcareous Grit. 



