302 GEOLOGICAL SURVEY OF THE TERRITORIES. 



apparently with a small sinus. Surface "with rather coarse, irregular 

 marks of growth. 



Length, 1.15 inches ; height, 0.55 inch ; convexity, 0.58 inch. 



This shell is much smaller than the typical species of Sowerby's genus 

 Pachymya, which it resembles in general habit and in the thickness of 

 its valves. Its ligament seems to have been short and rather deeply 

 inserted behind and between the beaks. I have not seen the hinge, 

 but judging from internal casts it would seem to be edentulous. Its 

 peculiar oblong form and squarely-truncated posterior margin give it 

 somewhat the aspect of a Saxicava, or a miniature Panojxca, but it 

 differs from these genera, in having thicker valves, and nearly or quite 

 closed margins. I am not sure that it belongs properly to the genus 

 Pachymya, though it must be nearly related to that genus, which seems 

 rather to belong to the MytiUdce than to be nearly allied to Pholadomya 

 or Panopwa, with which some have associated it. 



Specifically, the shell under consideration will be readily distinguished 

 from all of our other known cretaceous forms by its peculiar oblong, 

 truncated form and prominent umbonal slopes, concave flanks, &c. The 

 only other form known to me that approaches it in these characters is 

 Mr. Gabb's Eemondia furcata, which is a much larger shell, with more 

 regular undulations, a more prominent base in front of the middle, a 

 less narrowly-rounded anterior basal extremity, and a more obliquely- 

 truncated posterior margin. Our shell also seems not to have the hinge 

 characters of Eemondia. 



Locality and position : The specimen was given to Dr. Hayden at the 

 Salt Lake, and was found in that region, but he could not ascertain 

 the precise locality. It is almost certainly a cretaceous species. 



IN0CERA31US ALTUS. (MEEK.) 



Shell attaining a medium size, vertically, or a Jittle obliquely, subovate, 

 being in the adult higher than long, and widening from the hinge down- 

 ward; moderately convex; equivalve, very inequilateral; hinge very 

 short and ranging nearly at right angles to the longer axis in the adult, 

 but a little more oblique in young shells; anterior side straight, long, 

 and truncated vertically or nearly at right angles to the hinge, immedi- 

 ately in front of the beaks ; base regularly rounded ; posterior outline 

 forming a broad, somewhat oblique, gentle curve from the posterior 

 end of the hinge into the base; beaks nearly or quite equal, rising 

 little above the hinge line, pointed, obliquely incurved, and placed 

 immediately over the anterior margin. Surface of cast showing more 

 or less regular, rather obscure concentric undulations, and faint traces of 

 radiating markings, the latter probably not being defined on the exterior. 



Height, about 6.50 inches; length, about 4.90 inches; convexity, 2.70 

 inches; length of hinge, about 2.40 inches. 



This species belongs to the section of the genus that includes perna- 

 like forms ; that is, shells with their vertical diameter greater than their 

 atero-posterior, and with a short hinge ranging nearly at right angles 

 to the longer (vertical) axis of the valves, terminal or anterior beaks, etc. 

 It differs, however, decidedly from all of the described species of that 

 type with which I am acquainted, in being almost exactly equivalve, with 

 its beaks also very nearly if not quite equal. In size and general 

 appearance it reminds one of I. nobilis, Minister, as figured in Goldfuss' 

 Petref. Germ., PL CIX, Fig. 4 and 4a ; but it differs from that species, 

 not only in its equal beaks, but in having its truncated anterior side 

 straight instead of concave in outline, and its surface undulated instead 

 of merely striated. 



