644 SYSTEMATIC PALEONTOLOGY 



between the posterior carina and the hinge margin ; escutcheon much more 

 sharply denned in the young forms than in the adults ; anterior portion 

 of shell smoothly rounded, even nasute in the young; base line approxi- 

 mately horizontal; posterior dorsal and distal margins merging into one 

 another in the adults, the lateral margin squarely truncate in the young ; 

 external surface broadly corrugated in the umbonal region, the summits 

 of the obtuse ridges thus formed crowned with sharp laminar plates uni- 

 form in thickness throughout their extent, although the altitude attained 

 sometimes approaches a centimeter; laminae often broken away leaving 

 only a faint scar which is soon eradicated by exposure; the number of 

 processes thus developed rarely exceeding five; ventral portion of adult 

 shell evenly rounded and sculptured only with heavy growth lines and 

 crowded resting stages; ligament external, opisthodetic, seated upon a 

 short but rather stout nymph; hinge plate heavy, two cardinals in the 

 right valve, the anterior trigonal and placed opposite the lateral, the pos- 

 terior robust, obliquely elongated and compressed, feebly sulcated 

 medially; a stout rounded anterior lateral tubercle developed on the 

 ventral side of the hinge plate near the anterior cardinal ; posterior lateral 

 grooved, profound, the inner surfaces finely striated transversely; two 

 cardinals present also in the left valve, both of them posteriorly produced, 

 the anterior stout and feebly sulcated, the posterior laminar and united 

 with the basal margin ; anterior lateral sharp, trigonal with a deep pocket 

 behind it for the reception of the corresponding lateral in the right valve ; 

 posterior lateral elevated, produced; muscle impressions distinct, the 

 anterior excavated ; pallial line entire. 



The young of the species are subquadrate in outline and when fully 

 armed present a very different aspect from the cordate adults from which 

 the laminar plates have been broken away and all traces of them obliter- 

 ated. However, all the changes in outline and sculpture may be observed 

 in a single individual so that there is no doubt of the absolute identity 

 of the V. conradi and V. trigona. 



Even though there were, Morton's well-figured type is a fully adult 

 form with all the characters of the individual described later by Gabb 

 under the name of V. trigona. 



