40 G. F. MATTHEW : ILLUSTRATIONS OF 



tubercle ou the mesiau ridge, and also bear similar little tubercles near their ends ; these 

 tubercles on the mesian ridge and its secondaries are on the margin of a depressed sub- 

 circular area of the interior of the shell near the i\mbo ; the depressed area is traversed 

 near the middle by two delicate diverging arched ridges, which are about as long as the 

 lateral branches of the mesian ridge, and extend toward the lateral margins of the valve. 

 Behind these ridges, a stronger and broadly rounded ridge, limiting on each side the 

 depressed area at the umbo, extends more directly toward the lateral margins, and separ- 

 ates the flattened marginal part of the valve, described by Professor Hartt, from the more 

 tumid part at the umbo. There is a slight oval swelling of the mesian ridge near the 

 umbo, but scarcely comparable to a nail head ; at this part of the mesian ridge there is a 

 T-shaped branching of the ridge, which, in some examples, appears to be confluent with 

 the hinge line, as in AcrolheJe granulata, as figured by Professor "W. C. Brogger,' but in others 

 is separated from it by two minute pits, jvrst within the posterior border. These lateral 

 branches give the head of the mesian ridge a cruciform appearance. 



The A'entral valve in this species rises into a somewhat sharply conical umbo, which 

 is perforated behind the apex. There is a faint indication of a narrow area, included in a 

 wide triangular space behind the umbo, which is strongly striate externally, parallel to 

 the hinge line. The front slope of the valve is often marked in the anterior half by a few 

 faint, radiating undulations, or broadly rounded plica', which increase by intercalation ; 

 these folds are much more distinct on the inner than on the outer surface of the valve. 

 The foramen is about one sixth from the posterior side of the valve, and there is a shallow 

 depression on the inner surface of the valve from the foramen to the posterior margin. 



The interior of the ventral valve has the following features : — There are two pits in 

 the umbonal hollow in front of and close to the foramen ; they are deeper behind than 

 before, and are divided by a narrow ridge ; a comma-shaped groove on each side of the 

 umbonal depression extends forward opposite to the foramen on each side from the hinge 

 area ; the grooves are separated from the deeper part of the umbonal hollow by narrow 

 low ridges, and from their extremities fainter grooves of about the same length arch out- 

 w^ard toward the lateral margins. Just within the posterior margin there are four minute 

 pits, of which the two inner correspond to small tooth-like projections of the dorsal valve ; 

 the two outer ones are opposite the posterior ends of the comma-shaped grooves of the 

 umbonal depression. The posterior margin of the ventral valve has a narrow raised rim 

 for the reception of the dorsal valve. 



The inner surface of the A^alves is not perfectly smooth ; it is marked by concentric 

 undul ations of growth, and when viewed with a lens is seen to be diversified with 

 numerous, minute, shallow depressions. The ventral valve also has irregular, concentric 

 rows of minute pits or tubercles, especially near the margins, which are related to the 

 radiating plicae found on this valve. 



Sculplure. The surface of the ventral valve in this species is marked by four or five 

 concentric lines of growth on the outer and flatter part of the A^alve, and by finer and less 

 distinct lines on the inner and more tumid part ; similar lines are found on the dorsal 

 valve. The whole surface of the valves is also marked with a minuter sculpturing ; 

 towards the outer part of the valve this finer ornamentation is most distinct, and may be 



' Om Para(loxi(le.s .skifrene ved Krekling, Tab. iv. Fig. 11. 



