\6y 



area, its exposed foramen, its erect and elevated beak with angular sides, and 

 the frequent divarication of its ribs close to the margin.^' " He seems, how- 

 ever, to have laid a little too much stress upon some of the points above men- 

 tioned. Thus, for instance, the median sitni'al depression is not seldom 

 observed in the type species o{ Atrypa'tcticiilaris too. The flatness of the 

 margin also is not a peciiliarity confir.ed to A. desquaniata alonfe.'^ As A. re-- 

 ticiilaris is a form which has a very wide range of distribution^ both' 

 geologically and geographically it consequently shows the greatest varia- 

 tion in form and size of any fossil ever recorded. Tlie characteristics 

 observed in a small number of specimens therefore must not be over- 

 estimated. In this respect Davidson^^ had already remarked that A. desqua- 

 niata, which he provisionally regarded as an independent species, might be a 

 variety of A. reticularis Sow. A form which must be separated at least as a 

 variety, by the present writer, was named A. reticularis by Hall in the work 

 above cited — so closely are these two names related to each other. Kayser 

 in this way established the relations between these two forms, A. desqiiaiuatct 

 being Separated as a variety of A. reticularis. His description runs : — " Von 

 nahezu kreisformigen oder subquadratischem Umriss, breiter als lang, 

 Schnabel wenig gekrummt, so dass Area und Stieloffnung sichtbar bleiben, 

 Stirnrand gerade oder nur wenig nach oben abgelenkt. Falten starker 

 als bei der Hauptform ,Anwachsringe in grosserem Abstanden.'^" Thus a 

 part of the examples figured by Schnur as Tcrebratitla insquaviosa and those 

 described as T. z'onata, were classified under this variety. As Atrypa reti- 

 cidaris is a very variable species, plaeontologists may feel inclined to identify 

 A. desqna^i/iata with it. As far, however, as our present knowledge of these 

 forms is concerned, there seems to be a constant series of diversities between 

 them. The shell of A. desquamata is on the whole more oblong and thinner 

 than the other. The beak is less incurved and consequetitly the foramen is 

 not concealed in the former. These characteristics are recognized in the 

 material from China now under considei'ation, when compared witli the type 



i) Whideorne: - loc. cit. i 

 • , ■ i) Davidson:— op. ciuv p. '59; ' 1 



5) Kayser: — op. cit., p. 544. 18-71. * ' - ■ 



