450 MISSISSIPPIAN BRACK IOPODA 



side along the postero-lateral margins being very faint and difficult to 

 distinguish, the largest plications at the front margin measure about 1 

 mm. from center to center. The plications are crossed by concentric lines 

 of growth, several of which in the anterior half of the shell, are strongly 

 marked and wrinkle-like in the type. The shell substance punctate. 



Remarks. The present description of this species has been based upon 

 the holotype now preserved in the White Collection at the University of 

 Michigan, the only example to come under the observation of the writer. 

 In the original definition of the genus Acambona no mention of a foramen 

 in the pedicle valve is made, and no indication of such an opening is 

 shown in the illustration of the shell, and this supposed imperforate con- 

 dition of this valve was evidently the character upon which the genus 

 was primarily based. The type specimen shows a large opening in the 

 umbonal portion of the pedicle valve just back of the beak which is be- 

 lieved to be, without doubt, a large foramen. The margins of this open- 

 ing, however, are irregular and indicate a broken condition except for a 

 short distance where the natural border of the. foramen is clearly present; 

 because of this condition, White evidently interpreted it as an acci- 

 dental fracture of the shell. The remarkable degree of encroachment of 

 the foramen upon the umbonal region of the valve, so that it is entirely 

 out of contact with the apex of the delthyrium, thus allowing the apex 

 of the beak to remain sharply pointed, was probably another reason for 

 White's interpretation of the opening as an accidental break. With the 

 elimination of the imperforate beak of the pedicle valve, there remains 

 no external character of more than specific value to separate this shell 

 from members of the genus Eumetria. It has been shown by Hall and 

 Clarke, 1 however, that a calcareous pedicle tube upon the inner surface 

 of the pseudodeltidium, such as is present in Retzia and Hustedia, is 

 wanting in Eumetria, and it is stated by these same authors 2 that such a 

 structure is present in Acambona primu: Hall and Clarke, however, had 

 apparently never studied the holotype of the species, and a careful ex- 

 amination of this specimen has failed to demonstrate either the presence 

 or the absence of such a structure; under the circumstances the status 

 of the genus Acambona must remain doubtful. If there is no pedicle tube 

 present, it is altogether likely that this species is not generically 

 different from the shells included in Eumetria, in which case the name 

 Acambona must take precedence over Eumetria, having been proposed by 

 White in 1862. one year previous to the first usage of Eumetria by Hall. 

 But if a pedicle tube is really present, the genus is doubtless a good one. 

 Because of the extreme rarity of examples of the species, it may be long 

 before its true relations can be satisfactorily established, but for the 



1 Pal. N. Y., vol. 8, pt. 2, pp. 115-119. (1895.) 

 2Loc. c:t., p. 119. 



