G. R. Wieland— Protostegan Plastron. 



19 



thickens into a rounded and heavy ridge previously mentioned, 

 while the inner edge thins out as it passes beneath the posterior 

 extension of the hypoplastron. Anteriorly there are two lateral 

 grooves for the reception of the two posterior hypoplastral 

 continuations. This dovetailing makes a very firm joint 

 between these two elements. 



Concluding Remarks. — From the further study involved in 

 the reconstruction given herewith I conclude that the genus 

 Archelon which I have proposed is unnecessary. Archelon 

 ischyros therefore becomes Protostegaischyra. Certain differ- 

 ences I pointed out in the processes of the humerus and pro- 

 coracoscapular together with unusually robust form and in- 

 crease of size are of distinct specific but not generic value. I 

 may mention that Cope represented the ribs of Protostega as 

 free and that Hay with additional material considered this 

 possible. The rib expansions unquestionably united in gigas 

 by suture as in ischyra, as Professor Cope has since assured 

 me must have been the case. 



Cope regarded Protostega as the type of a new family which 

 he did not name, but with the idea that the plastral elements 

 were dorsal — an error much more easily made than one might 

 at first think — especially since some of the marginalia do 

 actually develop into thin dorsal spine set extensions. 



Of the generic and family relations of these important forms 

 I shall have more to say at a subsequent time. 



Measurements. 

 Tlie Entepiplastron. 



Extreme length (lateral) _ _ . 



Antero-posterior length _. 



Greatest breadth of alse 



Least breadth of alae . . _ _ _ 



Least circumference of alse 



Thickness at least circumference 



Greatest thickness in medial line'. 



Length of entoplastral process _ _ 



The Hyoplaston. 



Greatest antero-posterior length inclusive of spines 1 



Greatest oblique length inclusive of spines 1 



Greatest width inclusive of spines 1 



Greatest antero posterior length exclusive of spines .. 



Greatest width exclusive of spines 



Greatest thickness _ . - . . 



Length of interior spines *15 — 



Length of exterior spines *15 — 



•94 

 •45 ± 

 •28 

 •10 

 •21 

 •05 

 •06 

 •36 ± 



03 ± 



20± 



10± 



92 



70 



048 



25 



20 



