6 BRITTSII FOSSIL TRIGONIiE. 



known ; it is rare, and is unlike the usual forms attributed to T. signala. In 

 Yorkshire T. signaia^ var. Zietenii, is remarkable for a general regularity and uniformity 

 of aspect, both as regards the figure of the shell and the surface ornaments ; the costae 

 and the tubercles are slightly developed, so that usually the tubercles are distinct only 

 near the middle of the valve, and the costae become small or string-like, and disappear 

 or are attenuated as they approach the pallial border. This variety occurs rarely, near 

 Scarborough, at Cloughton, in the zone with Ammonites Blagdeni^ A. Braikenrid(/ii, and 

 A. Ilumphriesianus. 



In the South of England, in the Cotteswolds, T. signata, var. Zietenii, is known only 

 in a higher position, or zone of Ammonites Parlcinsoni^ where it is also a rare species, but 

 has a greater diversity of aspect in the figure of the shell, in its costae, in its tubercles, 

 and in the ornamentation of its area ; the costae are sometimes malformed or broken and 

 irregular ; the tubercles vary much in size and prominence ; the carinae may be either 

 prominent, or faintly defined ; and the transverse plications upon the area are either fine 

 and delicate, or coarse and irregularly disposed. In the Cotteswolds the tubercles upon 

 the costae are comparatively large, and continue rounded even to the pallial border; the 

 area has its ornamentation almost evanescent both in its carinae and in its transverse 

 plications, becoming almost steep and narrow, and the shell acquires greater convexity. 

 The Oxfordshire variety Zietenii is generally and constantly smaller than that of the 

 Cotteswolds and somewhat more lengthened posteally ; the costae are equally numerous, 

 but have the tubercles smaller and numerous, which imparts a neater aspect to the shell ; 

 the individual peculiarities are also less conspicuous and rarely exceed those exhibited in 

 the forms on Suppl., PL I, figs. 4 and 5, which have been selected to exhibit malformations 

 of the rows of costae upon the middle of the valves ; but the differences of ornamentation 

 upon the area are comparatively trifling. 



The bed of white limestone (Upper Trigonia-grit) is the only stratum which has 

 produced T. sigtiata in the Cotteswolds. My experience of twenty-two years' residence in 

 that district, together with frequent subsequent comparisons of its fossils, induce me to 

 regard the Upper Trigonia-grit as affording specimens greatly inferior to those of the 

 sandy limestones of Oxfordshire. The condition of preservation of the more minute and 

 dehcate features is usually less satisfactory. These remarks will more especially apply 

 to the present variety, a typical shell of the Inferior Oolite, everywhere rare, but espe- 

 cially so in the Cotteswolds, where not more than one or two really fine specimens can 

 be expected to fall to the lot of any collector, however extended in time may be his 

 search for them. Even in Oxfordshire, where the species has occurred in greater numbers 

 in the sands and sandy limestones of Hook Norton and Rollright Heath, it has of late 

 years become somewhat rare. Mr. Stutterd, of Banbury, was fortunate enough to collect 

 numerous fine specimens which now adorn the University Museum, Oxford, a success 

 which has not subsequently been rivalled by any other person, and the species is no 

 longer common. 



