182 



CONCHIFERA. 



ed, divergent, two in the riglit valve transversely grooved on 

 both sides ; the grooves regularly marked, each forming the 

 segment of a circle ; four teeth in the left valve grooved in one 

 side only, but those alternately in pairs ; consequently the four 

 teeth of this valve receive within their grooved sides the two 

 teeth of the right valve ; two principal muscular impressions, 

 the lateral ones very distinct, one of which is situate close to 

 the superior end of the cardinal tooth, and a little behind it ; 

 the other somewhat more distant, with a minute one between 

 it and the cardinal tooth ; pallial impressions almost entire ; 

 ligament marginal, thick, rather short, and external. 



1. Trigonia costata. — The Kibbed Trigonia, pi. LXXV. 

 fig. 4. 



F. costata. Parkinson, Org. Rem. III. pi. 12, fig. 4. 

 Sowerby, I. p. ] i)5, pi. 85. 



Trigonal, posterior side somewhat ventricose, with a series 

 of transverse, elevated, curved, smooth ribs, and smooth inter- 

 costal spaces ; anterior side large, divided from the flattened 

 anterior by an elevated, longitudinal, obliquely curved, cren- 

 ated rib, reaching from the beak to the margin, and projecting 

 a little beyond it in some specimens ; two additional equi- 

 distant longitudinal ribs divide the side ; several longitudinal, 

 raised, crenated, thread like striae occupy the intercostal spaces; 

 beaks nearly central, and sub-acute. 



A widely difl'used species, existing through various forma- 

 tions, viz. : the Oxford Clay, Osmington, Dorsetshire ; the 

 Ooral Rag, Malton and Steeple Ashton ; the Great Oolite, 

 White Nab, Stonefield; and the Inferior Oolite, Limpley 

 Stoke and Cotswald Hill. 



2. Trigonia elo.ngata. — The Elongated Trigonia, pi. 

 LXXIL* fig. 13. 



T. elongata. Sowerby, V. p. 3,0, pi. 431. 



Elongated ; sub-triangular, slightly oblique, and gibbose ; 

 beaks nearly central, a little reflected ; anterior side moderately 

 arcuated, with a series of transverse, elevated, smooth ribs, 

 with wide intervening furrows ; posterior side separated from 

 the anterior side by a longitudinal curved furrow and a crenu- 

 lated rib, with two additional distant ribs ; the broad inter- 

 vening spaces being covered with wide-set, longitudinally 

 curved stri;e, crossed by waved transverse striae, producing a 

 scaly appearance. 



In the Lower Greensand, Ashford, Kent, and the Portland 

 Stone, Dorsetshire. 



3. Trigonia ddplicata. — The Two-plaited Trigonia, pi. 

 LXXII.* fig. 14. 



T. diiplicata. Sowerby, III. p. 63, pi. 237, figs. 4, 5. 



Transversely and obliquely oblong ; somewhat compressed ; 

 anterior side large, with small, arcuated, curved ridges ; and a 

 strong tuberculated curved rib separating it from the posterior 

 aide, which is small, and covered with obliquely transverse 

 narrow furrows ; beaks rather large and prominent ; basal 

 margin crenated. 



The Inferior Oolite, Glaizedale, Yorkshire. 



4. Trigonia pennata. — The Winged Trigonia, pi. LXXII.* 

 fig. 9. 



T. pennata. Sowerby, III. p. 64, pi. 237, fig. 6. 



Obliquely oblong, and considerably arcuated ; the anterior 

 side arched, with from ten to twelve elevated, obliquely curved 

 ribs, which are striated on their fronts ; with smooth inter- 



costal spaces ; posterior side flattened, concave, and divided 

 into two lobes by a longitudinal furrow, on each side of which 

 a series of tuberculated ribs diverge downwards ; beaks sub- 

 acute, a little bent ; basal line cleft. 



The Greensand, Teignmouth, Devonshire. 



5. Trigonia STRIATA. — The Striated Trigonia, pi. LXXII.* 

 fig. 15. 



T. striata. Sowerby, III. p. 63, pi. 237, figs. 1,2,3. 



Elongated, sub-triangular, inflated ; posterior side with large, 

 thick, crenated, transverse ribs ; anterior side with numerous . 

 oblique, waved stri;c, subdivided by a longitudinal furrow ; 

 basal line of the posterior si<le waved ; beaks sharp-pointed ; 

 beneath them a longitudinal, lanceolate lunule. 



The Inferior Oolite, Dundry, Yorkshire, and Cotswold 

 Hill. 



6. Trigonia angulata. — The Angnlar Trigonia, pl.LXXV. 

 fig. 1. 



T. angulata. Sowerby, VI. p. 9, pi. 508, fig. 1. lb. T. 

 davellata, var. I. p. 197, pi. 87, lower figs. 



Transversely sub-triangular and elongated ; rather convex ; 

 anterior side sub-arcuated, and abruptly sloping ; surface with 

 transverse, curved, raised ribs, each of which, on reaching the 

 disk, changes into a continuous series of nodules ; posterior 

 side nmch produced, and divided from the anterior by a curved, 

 longitudinal, crenated rib, with the extremity considerably 

 acuminate and truncated ; crossed by transverse, wide-set 

 stria; ; the upper edge concave. 



The Inferior Oolite, Nunney and Blue AVick. 



7. Trigo.nia al.eformis. — Th3 Wing-shaped Trigonia, pi. 

 LXXV. fig. 7. 



T. alwforntis. Parkinson, Org. Rem. III. p. 176, pi. 12, 

 fig. 9. Sowerby, III. p. 27, pi. 215, figs. 1, 2, 3, 4. 



Sub-triangular ; wing-shaped ; anterior side arcuated, and 

 occupying the greater part of the valve ; with many trans- 

 verse, tuberculous ribs, which become smooth, plain, and nar- 

 row towards the inner side ; intercostal spaces smooth ; pos- 

 terior side lengthened into an acuminated, sub- truncated beak, 

 and separated from the other portion by a broad, curved, flat- 

 tened ridge, extending from the beak, and rapidly widening as 

 it approaches the basal margin ; beyond the ridge is a flat 

 space, crossed by oblique, flat ribs, divided by very narrow 

 furrows ; beaks acute, and much incurved. 



In the Gault, Vale of Wardour ; the Lower Greensand, 

 Broughton, Blackdown, and Lyme Regis. 



8. Trigonia imbricata. — The Imbricated Trigonia, pi. 

 LXXV. fig. 2. 



T. imbricata. Sowerby, VI. p. , pi. 507, figs. 2, 3. 



Triangular ; compressed ; anterior side with five or six trans- 

 verse, imbricated ribs ; posterior side obliquely truncated, with 

 oblique, transverse, flattened ribs. 



The ribs upon this shell resemble a series of terraces, situate one 

 above another. 



The Great Oolite, Anclifl". 



9. Trigonia quadrata. — The Squarish Trigonia, pi. 

 LXXV. fig. 5. 



T. quadrata. Sowerby, Geo. Trans. 2d Ser. IV. p. 342, pi. 

 17, fig. 12. 



Sub-quadrangular ; compressed ; anterior side short and 

 much rounded ; posterior side broad and truncated ; beaks ob- 



