INTRODUCTION. 



1st. SCAPHOIDE^E. 



This section, of which the type is the Trigonia navis of Lamarck, was constituted by 

 Agassiz to include species in which the form is usually triangular, having a remarkable 

 straightness or truncation of the anterior border ; the other two borders converge and 

 terminate posteriorly in an extremity which is more or less truncated or obtuse. The 

 area is nearly smooth, bounded above by an inner, and beneath by a marginal carina; 

 these are always small and sometimes evanescent. The costse form two series, the larger 

 or posteal series pass almost perpendicularly downwards from the marginal carina to the 

 pallial border, the smaller or anteal series have their general direction horizontal or at 

 right angles to the other series ; they are short and occasionally, as in T. navis, 

 terminate posteally in a large varix. Agassiz described and figured five species, three of 

 which are Jurassic and two Cretaceous ; the present Monograph adds four species, all of 

 which are from the Lower Oolites, and one is new. These are the following : — T. duplicata, 

 Sow;, T. gemmata, Lye, T. Bathonica, Lye, and T. recticosta, Lye. 



2nd. — ClavellatjE. 



An essentially and almost exclusively Jurassic section have their sides ornamented 

 with tuberculated costse in rows, which are either concentric or oblique ; the tubercles in 

 the rows usually become indistinct or cord-like as they approach the pallial border, and 

 the last-formed rows of costse are often interrupted or broken and obscured by numerous 

 plications of growth. The area, which is well separated, is bounded by two tuberculated 

 carinas, and has usually also a smaller mesial oblique carina bordering upon a furrow, 

 thus dividing the area into two parts. The escutcheon is always plain, as in the 

 Undulates ; it is depressed and is usually well circumscribed by the inner tuberculated 

 carina. Few species have the rows of costse regular and symmetrical over the whole of 

 the side, or adjacent to the pallial border ; most commonly there is some irregularity in the 

 rows near to their anteal terminations ; some individuals of certain species have this 

 irregularity so considerable that the tubercles become altogether confused and crowded, 

 in other instances they become absorbed in the longitudinal plications, which form 

 squamous elevations. It rarely happens that this section has the granulated tegument 

 preserved ; and it may be remarked that the same clays and shales which have preserved 

 this feature in the Costata have failed to do so in the Clavettata. The species of the 

 Clavellatm are very numerous, and in their habits they were usually gregarious, but it 

 seldom happens that any one species can be traced over any considerable geographical 

 area. The interior of the valves have near to their posteal slope a prominent lengthened 

 ridge, which indicates the position and separation of the siphonal currents. The following 

 species, twenty-eight in number, are British examples of this section : — T. clavellata, 



