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NEW YORK STATE MUSEUM 



an umbo that is situated near the anterior margin. Where crusta- 

 ceans have a similar system of lines they also nearly always display 

 finer markings, especially fine networks of raised lines, either 

 between the ridges, as in Estheria, Emmelezoe, etc., or on the inside 

 (Anatifopsis). This finer sculpture is so characteristic of crusta- 

 cean valves that one naturally always looks for it as soon as a valve 

 is suspected of being the carapace of a crustacean. No trace of 

 this fine sculpture is observable in any of the Discinocarina. The 

 fine longitudinal lines along the median line of Spathiocaris are of 



a different character. 



Another difference that 



seems of some significance is 

 seen in the mode of accretion 

 at the edge of the valve or in 

 the direction of the growth 

 segments. In lamellibranchs, 

 brachiopods and crustaceans 

 which grow from an umbo by 

 accretion at the edge of the 

 shell, the new portions are 

 added from the inside of the 

 shell, the new segments there- 

 fore pass beneath it and the 

 older growth-lines always 

 overlap the younger ones. In 

 an aptychus, however, which 

 is formed within the conch of 

 a cephalopod, the accretion 

 takes place so that the new 

 material is added at the outer 

 edge sloping upward against 

 the old shell (see text fig. 34) ; the new shell segments thus pass not 

 inward under the old ones but upward and outward. This, however, 

 is precisely the structure that We also find in Spathiocaris, and, as 

 far as the writer is aware, in the other genera under, suspicion; in 

 fact in all Discinocarina and, judging from the figures, very typically 

 in Dipterocaris vetustus and, from inspection, in our 

 species of Dipterocaris. It is thereby, however, to be remembered 

 that the mode of accretion described above of the Discinocarina is 

 that of the aptychi, while these valves have been compared with the 

 anaptychi. The anaptychus (see Zittel-Eastman, v. 1, p. 545) is, in 

 distinction from the aptychus, a single plate which is invariably 



Fig. 34. Aptychus of Aspidoceras; 

 first figure is a view of the concave 

 interior side, Showing the concentric 

 lines ; the second figure is a view of 

 the convex ventral side, showing the 

 punctate surface; the third figure is a 

 transverse section below the middle of 

 the first figured specimen and showing 

 the direction of the accretions. (From 

 Steinmann) 



