Br. G. F. Matthew— The Oldest Siphonotreta. 



PEOTOSIPHON, n. subgen. 

 Protosiphon Kempandm, n.sp.^ 



Protosiphon Kempanum. — 1. Interior of the ventral valve, showing scars of central 

 and lateral muscles, opening of the siphon, vascular trunks, etc. 2. Interior of the 

 dorsal valve, showing position of the central and lateral muscles, median ridge, 

 vascular trunks, etc. 3. Section of the beak of the ventral valve, showing the 

 passage for the pedicle. 4. Umbo of the ventral valve viewed from above, showing 

 the Schizambonal furrow. — All the figures are magnified f , except No. 3, which is 

 enlarged f- — N.B. The large figures are somewhat idealized, as some features (e.g. 

 the vascular trunks) are supplied from examples other than the two which formed the 

 basis of these drawings. 



Shell substance calcareo-corneous. Outline of the dorsal valve 

 somewhat oblately orbicular; outline of the ventral valve similar, 

 but with an obtusely pointed beak. Both valves moderately convex, 

 and marked at the hinge area by transverse ridges of growth. The 

 ventral valve has a depressed channel on the back, beginning at the 

 beak and ending forward in a tube buried in the substance of the 

 shell, and terminating inwardly in the visceral cavity by a minute 

 opening. 



The dorsal valve does not differ much from the ventral, except in 

 the absence of a projecting beak, and in being more tumid in the 

 posterior half. Both valves have a broad, shallow sinus towards 

 the front, and so are there straighter than in other parts of their 

 circumference. 



The edges of both valves are flattened on the underside, and 

 thickened at the cardinal line, where thej' are traversed by arched 

 ridges and furrows, which may serve the purpose of articulation. 

 A depression similar to a pedicle groove traverses the cardinal area 

 of the ventral valve on the axial line ; and a low tubercle holds 

 a similar position on the dorsal valve. 



Smdpture. — The surface of the shell is marked by fine lines, 

 concentric to the umbo, and at intervals by stronger growth-lines. 



1 Dedicated to Dr. J. F. Kemp, Professor of Geology at Columbia College 

 University, New York, well known for his work on the Archa3au and Cambriaa 

 Geology of the State of New York. 



