108 



PALEONTOLOGY OF NEW YORK. 



JAntutrssonia sagittatia. 



Aflei- llAVIDSON. 



Fig. 60. Internal cast of peiliclc-valve. 

 Fig. 61. Internal cast of brachial valve. 



that the generic character.s arc not only very distinctly defined, but are of such 

 a nature as to remove all these forms widely from Obolella and its immediate 

 allies. Tliis is seen in the character of the muscular impressions alone ; the 

 absence of the great lateral scars, the broad, composite centrals, and the deep 

 pedicle-pit of the ventral valve, all of which are indicial features of the obolel- 

 loid genera ; furthermore there is no evidence of any cardinal area or anything 

 more than a somewhat flattened margin of 

 contact in either valve. On the other hand 

 there are present (in L. sagittalis) conspicuous 

 cardinal scars in each valve, of similar char- 

 acter to the posterior adductors of Crania (as 

 suggested by Mr. Davidson, lac. cit., p. 340), 

 which, in L. transversa and L. misera, take 

 the form of stout tubercles or bosses, situated just within the posterior margin; 

 and a mammiform or subtrihedral apical tubercle, which is perforated by a 

 minute foramen (see Plate III, figs. 35, 37-39, 43). This feature was not 

 represented in Mr. Davidson's figures of Obolella sagittalis, but was first pointed 

 out by Linnarsson {loc. cit.), and is clearly seen in well preserved specimens. 

 The character of the pedicle-aperture is of itself of sufficient significance to 

 remove these species from the vicinity of Obolos and Obolella, where this 

 opening is always a groove on a well defined cardinal area, and to endorse its 

 association with Acrotreta, Acrothele, Schizambon, and the other allies of 

 Siphonotreta. The posterior submarginal tubercles or bosses have undoubtedly 

 served as fulcra, probably of the adjustor rather than of the adductor muscles, 

 acting to keep the valves in place, and they may be compared to the articular 

 bosses in the genera Spondylobolus and Barroisella. 



The brachial valve has a less conspicuous development of these tubercles, but 

 they are distinctly seen in L. transversa, L. sagittalis and L. Taconica. Between 

 them lies a strong axial septum, which is broadened over the central portion 

 of the shell, and is accompanied for most of its length by a shorter and narrower 

 ridge on either side, from each of which it is separated by a sharp furrow. 



The substance of the shell, in all the specimens which have passed under our 



