69 



Towal■d^s the posterior end', the ribs are transverse and cross the valves 

 in a radiating manner, the radii being directed backwards. At their 

 junction with the outer border of the hinge area, these posterior costs? 

 are narrow above and widen symmetrically to the mai'gin below or 

 behind. 



The posterior area is ribbed longitudinally ; but the costa? are curved 

 and follow its general outline. At first they are very narrow, and attain 

 their maximum width at the point farthest from the beaks. In some 

 specimens the ribs which traverse the valves behind bifurcate with those 

 on the posterior area, and at the extreme tip of the beaked end the 

 directiori of both is not fur from parallel. 



The sculjjture of this distincth- characterized species is subject to con- 

 siderable variation. In one specimen the transverse ribs which proceed 

 from the anterior margin, are bent into a series of zigzags before they 

 take their final upward turn. In another distorted example the same 

 ribs traverse fully two-thirds of the shell before they betid upwards, and 

 their angles lie in the direction of a line drawn obliquely from the beaks 

 to the base of the posterior end. In the majority, the longitudinal and 

 transverse ribs occupy each about one-half of the surface, and the angles 

 of those which so suddenly alter their course are j)laced in the direction 

 of an obliquely concave line which might be drawn from the beaks to a 

 little behind the middle of the base. Again, in one instance the costse 

 on the posterior area bifurcate distinctly with the transverse ribs on the 

 main body of the shell, in other examples the latter are truncated by the 

 former. In all cases, however, the ribs in front, though they trend up- 

 wards, and their course is more or less broken, are nearly longitudinal ; 

 on the posterior area they are decidedly so, while on the beaked post- 

 erior end and on the hinder part of the umbonal region they are trans- 

 versely radiating. 



Grreatest length of the specimen figured, twenty-one lines ; height in 

 the centre, behind the beaks, nine and a half Hues : thickness, nine lines. 



Seven specimens were collected, some of which are a good deal 

 distorted. 



A curious little Trigonia of the Scaphoid group, easily recognized 

 by its very j^eculiar sculpture. The only species with which it 

 might be confounded is the Trigania Van of Sharpe * from Secondary 

 and probably Jurassic rocks in South Africa. The shape of these two 



* " Transactions of the Geological Society of London." Second Series. Vol. VII., page 194, Plate 

 XXII., fig. 5. 



