CYRTIA 383 



The species which is here recognized is based upon a single specimen of 

 an incomplete pedicle valve which differs from the genotype in the greater 

 development of the dental lamellae, in the plicated lateral slopes of the 

 valve and in the absence of the foramen through the pseudodeltidium. 

 The specimen resembles the members of the genus Pseudosyrinx but 

 lacks the delthyrial plate and the punctate shell structure, and possesses 

 a convex pseudodeltidial covering of the delthyrium. It seems to agree 

 with Cyrtia more closely than with any other genus, hence its reference 

 to that genus in this place. 



CYRTIA INEXPECTANS n. sp. 

 Plate LXIII, Figs. 1-4 



Description. Shell of medium size, the hinge-line a little shorter than 

 the greatest width of the shell, the cardinal extremities rounded. The 

 species is known only from a single incomplete pedicle valve whose di- 

 mensions, so far as they can be determined, are: width along hinge-line 

 32 mm., height of cardinal area 14 mm., length of pedicle valve from beak 

 to front margin -f-21 mm. 



Pedicle valve subpyramidal in form, the surface sloping from the umbo 

 to the lateral extremities in nearly straight lines which meet at the umbo 

 in an angle of about 110 degjrees, to the anterior margin the surface is 

 gently convex ; mesial sinus non-plicate, shallow, scarcely more than a 

 median flattening of the valve ; the beak pointed, curved so as to be di- 

 rected at nearly a right angle to the cardinal area; cardinal area high, 

 nearly flat below, becoming gently concave towards the apex beneath the 

 beak, the lateral margins not sharply denned, the surface rounding into 

 that of the lateral slopes ; delthyrium narrowly triangular, at least twice 

 as high as its width at the base, entirely covered by a convex pseudo- 

 deltidium through which there is no foramen ; each lateral slope marked 

 by 12 or 14 simple, depressed, radiating plications which originate along 

 the cardinal margin and become very faint towards the cardinal extremi- 

 ties. Internally the dental lamella? are strongly developed, extending 

 anteriorly from the beak to the anterior margin of the shell so far as it is 

 preserved, they diverge from the beak at an angle of about 26 degrees, 

 nearly following the lateral margins of the mesial sinus ; the muscular 

 scars are only faintly marked and are not divided longitudinally by a 

 median ridge or septum. 



The surface markings of the shell, aside from the plications, so far as 

 shown in the single specimen, consist of a few concentric lines of growth ; 

 so far as can be seen the shell structure is impunctate. 



Remarks. The specimen upon which this species is based is unique. It 

 resembles some of the shells which have been described under the new 

 genus Pseudosyrinx, especially P. missouriensis, but it differs from that 



