ABT. 22 MITEOSPIEA, A NEW GASTEEOPOD KIBK d 



rina by Wilson but does not seem to be a direct derivative. It seems 

 more probable that it was derived from a MacluHtes like form b}' 

 way of some such intermediate stage as Mitrospira or belongs to 

 some other genetic line. In section an adult whorl of Pallweria has 

 a subquadrate or irregularly polygonal outline, Wilson describing 

 the form as having " six outstanding angles formed by the carinae." 

 Besides differing from Mitrospira in the sharply angulated mature 

 whorl, Palliseria is easily distinguished by its slightly developed 

 apertural notch. In this connection it is of interest to note two 

 racial groups within the genus Maclw'ites as now defined. In one 

 there is a pronounced apertural notch, and in the other the notch 

 is inconspicuous or wanting. It may well be that two distinct 

 generic types are represented, but as a rule the material available 

 for study does not show the apertural margin or the growth lines 

 clearly, and at present it would be inadvisable to make such a separa- 

 tion. PalUserim is found well up in the Upper Ordovician, 



Genotype. — The genotype and at present only known species is 

 Mitrospira longweUi., new species. 



HoHzon and locality. — The genus is known in the upper portion 

 of the Pogonip limestone (of Chazyan age) in Nevada and Cali- 

 fornia. Dr. E. O. Ulrich informs me that he has at least two species 

 of Mitrospira., one from Canada and another from Oklahoma. The 

 Oklahoma species occurs in the lower part of the Simpson formation. 

 At Phillipsburg, Canada, a species has been collected in beds con- 

 sidered by Ulrich as correlating with the uppermost Beekmantown 

 beds as shown at Fort Cassin, or possibly of even later age. 



MITROSPIRA LONGWELLI, new species 



This species attains a very large size as compared with most gas- 

 teropods of equivalent age. An adult individual but not of maxi- 

 mum size has a height of 55 millimeters and a breadth of not less 

 than 80 millimeters. An adult individual has from seven to eight 

 whorls that increase very rapidly in size from the apex to the aper- 

 ture. In the specimen noted above the whorl at the aperture has a 

 height of approximately 40 millimeters and a maximum breadth of 

 20 millimeters. The lower side of the shell has typically a broadly 

 expanding subconical profile. The height of the spire is somewhat 

 variable, due to a greater or less amount of overlap of the whorls on 

 those preceding. On Plate 3, Figure 5, is shown the highest spired 

 specimen seen, in which the height and breadth of the specimen are 

 approximately equal. This specimen is an extreme variant and is 

 doubtfully referred to M. longioelli. 



The whorls, of which there are seven to eight in an average adult 

 specimen, increase rapidly in size with the growth of the animal. 



