184 



CONCHIFERA. 



Nltul.v. 



curved, and approximating ; back gradually sloping from the 

 beaks ; basal line gently curved ; surface with regular, trans- 

 verse, shallow furrows, or lines of growth, and a few short, 

 oblique, shallow ones crossing in a lozenge manner at the an- 

 terior side. 



The Greensand, Staple Hill and Devonshire. 



20. Trigonia d^dalea. — The Handsome Trigonia, pi. 

 LXXVI. fig. 21. 



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

 fig. 6. Sowerby, I. p. 198, pi. 88. 



Oblong-ovate, trigonal ; a longitudinal, tuberculated ridge, 

 dividing the valves into nearly equal halves ; both sides an- 

 gular near their centre, the anterior one gradually rounded 

 both above and below the angle ; posterior side slightly hol- 

 lowed beneath the beaks, and thence a little arcuated till it 

 reaches the centre, below which there is a double flexure ; 

 beaks small and pointed ; anterior side with many series of 

 large tubercles, set in arcuated ridges ; posterior side with 

 a series of less regular ones running downwards from the side 

 towards the central ridge. 



The Lower Greensand, Parbani and Blackdown. 



21. Trigonia incurva. — The Incurved Trigonia, pi. 

 LXXVI. fig. 42. 



T. incurva. Sowerby, Geo. Tr. 2d Ser. IV. p. 34.7, pi. 

 22, fig. 14. Bennet, "Wiltshire, Foss. pi. 18, fig. 2. 



Transversely and obliquely longitudinal, its width nearly 

 double its length, convex ; posteriorly flattened ; surface tu- 

 berculate, set in curved series. 



The Portland Stone, Portland ; Vale of Wardour and 

 Swindon. 



22. Trigonia impressa. — The Impressed Trigonia. 



T. impressa. Sowerby, Zool. Journ. III. p. pi. 11. fig. 1. 



Sub-triaugular, anterior side rounded ; posterior side some- 

 what truncated, beaks obtuse ; surface with a series of con- 

 centric papillose ribs. 



The Great Oolite, Stonefield. 



23. Trigonia literata. — Lettered Trigonia, pi. LXI.*** 

 fig. 21. 



T. literata. Phillips, Geo. York, I. pi. 14, fig. 11. 



Sub-triangular, sub-conic, moderately convex ; anterior side 

 rather abruptly sub-truncated ; posterior side concave above, 

 considerably produced, and somewhat acuminated below, se- 

 parated from the other portion by a longitudinal, linear row of 

 obtuse tubercles, which emanate from the beak, and with an 

 obscure longitudinal row of pustules in its centre, obliquely 

 crossed by lines of growth ; anterior portion of the surface 

 with a series of very strong, rugged, longitudinal ribs, which 

 rapidly thicken as they descend, emanating from the pus- 

 tular division, which, on reaching the centre of the valve, 

 suddenly turn upwards at an acute angle, in the form of the 

 letter F, and again turn to the anterior margin ; all the ribs 

 are crossed by irregular rough stria? ; beaks acuminated and 

 incurved ; lunule, large, and bounded by a margin of trans- 

 versly elongated pustules : basal line convex. 



The Lias, Robin Hood's Bav, Yorkshire. 



Tribe III.— ARCACEA. 

 Shells provided with numerous small primary teeth, dis- 

 posed in a straight or interrupted line in each valve. 



Genus VII.— NUCULA.— Za«iarc/L-. 



Shell equivalve, inequilateral, transverse, oval, trigonal, or 

 oblong ; generally covered with a strong epidermis ; hinge 

 linear, narrow, divided into two parts by an oblique, produced, 

 nearly central pit, which is destined for the reception of the 

 ligament ; the one anterior, and the other posterior ; lateral 

 teeth on each side numerous, acute, elevated, somewhat re- 

 curved, those of the opposite valves locking into the inter- 

 vening spaces ; umbones contiguous, and not separated by an 

 intervening area ; two simple, muscular impressions ; mantle 

 impression destitute of a sinus. 



1. Nucula ovalis. — The Oval Nucula, pi. LXXVI. fig. 

 33. 



N. (?) ovalis. Murchison, Silur. Syst. pt. II. p. 609, pi. 5, 

 fig. 8. 



Shell transversely ovate, smooth, rather convex ; beaks sub- 

 acute, and placed near the anterior side ; length about four 

 lines, breadth five lines. 



Tiie Upper Ludlow Rock, Trewerne Hills on the Wye, 

 Radnorshire. 



2. Nucula l./evis. — The Smooth Nucula, pi. LXXVI. fig. 

 22. 



N. Icevis. Murchison, Silur. Syst. pt. II. p. 635, pi. 22, f. 1 . 



Shell oval, transversely elongated, smooth, ventricose ; beaks 

 large, acute ; length three-eighths of an inch, breadth two- 

 eighths. 



Found in Black Schist, in the Lower Silurian Rocks, Pen- 

 sarn, near Caermarthen, Wales. 



3. Nucula Cobboldi^. — Cobbold's Nucula, pi. LXXVI. 

 fig. 55. 



N. Cobboldia: Sowerby, II. p. 177, pi. 180, fig. 2. 



Transversely obovate, convex ; posterior side very short, 

 with numerous, shallow, smooth, zigzag furrows, diverging 

 over the sides ; space between the teeth elongated and deej) ; 

 margin entire. 



The Mammiferous Crag, Bramerton, and the Red Crasj. 

 Sutton. 



4. Nucula lanceolata. — The Lance-shaped Nucula, pi. 

 LXXVI. fig. 55. 



N. lanceolata. Sowerby, II. p. 178, pi. 180, fig. 1. 



Transversely lanceolate and ovate ; width double the length ; 

 sides nearly equal, the antciior very slightly the largest; pos- 

 terior side a little pointed ; beaks a little produced ; surface 

 smooth ; hinge with a deltoidal concave space ; margin entire ; 

 substance of the shell strong. 



The Red Crag, Bawdsey. 



5. Nucula deltoiuea. — The Deltoidal Nucula, pi. 

 LXXVI. fig. 51. 



N. deltoidea. Sowerby, VI. p. 103, pi. 554, fig. 1. 



Triangular, ventricose ; anterior side short, and rounded ; 

 posterior side obliquely truncated, flat, and pointed ; generallv 

 smooth, but sometimes longitudinally striated towards the an- 

 terior margin. 



The Upper Marie, Isle of Wight, and Bagshot Sands, Shap- 

 ley Heath. 



6. Nucula inflata. — The Inflated Nucula, pi. LXXVI. 

 figs. 11, 12. 



A', inflata. Sowerby, VI. p. 103, pi. 554, fig. 2. 



Almost globular ; the posterior side small, and a little pro- 



