74 University of Texas Bulletin 



branch ; in shape it resembles the lateral saddles but is much smaller. 

 It is high and bulges on both sides, in the middle. The adventive lobe 

 A between the two branches of the external saddle has a simply 

 rounded bottom.^ 



The two lateral saddles have in general the form of asymmetrical 

 trilobate leaves ; that is to say, near the middle they bear lateral pro- 

 tuberances which are directed obliquely above. In .the first lateral 

 saddle, the protuberance on the siphonal side has a higher position 

 than that on the umbilical side; in the second lateral saddle the condi- 

 tions are reversed. The first auxiliary saddle is similar to the second 

 lateral ; in the second and third auxiliary saddle the protuberances are 

 greatly reduced, and the rest of the auxiliary saddles are more or less 

 tongue-shaped and somewhat constricted near the base. 



From the first lateral until the last auxiliary the height of the saddles 

 diminishes gradually. 



Dimensions : 



Diameter 41.8 mm (1) 



"Width 9.2 0.21 



Height of the last whorl 25.5 0.61 



Diameter of umbilicus 1.3 0.03 



Relations to other species: 



The shape and sculpture of our species are very similar to those of 

 M. Orbignyana Vern.^ the latter one showing also in the younger 

 whorls tubercles on the keels, while in the later whorls these become 

 entirely smooth and sharp. Especially the figures given by Tchernow 

 demonstrate how similar the sculpture is. But the suture in both 

 species is entirely different, M. Orbignyana having generally tongue- 

 shaped saddles, while those of our species resemble trilobate leaves. 

 The siphonal branch of the external saddle is in M. Orbignyana much 

 less asymmetrical than in M. Whitneyi. 



'We have treated A and os as two diferent adventive lobes, following the example 

 of Noetlittg; but it seems to me much more natural to consider these lobes together 

 as A, divided by an adventive saddle. In M. Orbignyana, Noetling considers as A 

 what would correspond to a rudimentary adventive lobe and the adventive lobe A 

 if the dividing saddle were not so extremely small. Genetically there is certainly 

 no diflference. 



"Verneull, Geol. of Russia, II, p. 375, pi. 26, fig. 6. 

 Karpinsky, Amm. d. Artinsk-Stufe, p. 35, pi. 2. fig. 1 a-n. 

 Tchernow, L'Etage d'Artinsk, p. 367, pi. 1, fig. 8 a-c. 



