NEW OSTRACODERM— 0RVIG 15 



specimens exhibiting simultaneously both the immature and mature 

 t3^pes of ornamentation) . This is surprising in view of the condition 

 of the exoskeleton in a great many other lower vertebrates where the 

 dentine tubercles belonging to consecutive generations certainly may 

 show var^ang degrees of complexity and hence may differ to some 

 extent from each other {sqq, e. g., 0rvig, 1957, p. 388), but neverthe- 

 less are alwaj's basically similar in their histological structure. 



The middle, vascular layer is thick, and its vascular canals are 

 UTegularly distributed except in some of the polygonal plates of the 

 carapace where they may, occasionally, show a more or less distinct 

 "transsutural" arrangement near the marginal faces (see Delpey, 

 1942, pp. 52-53; 0rvig, in MS., a). Ontogenetically this layer arose 

 as a system of thin, bony trabeculae separated by fairly wide vascular 

 spaces, but later on it became rather compact (more so, in fact, than 

 the corresponding layer of the exoskeleton in most post-Ordovician 

 Heterostraci) by the deposition of thick-walled primary osteons on 

 the margins of the trabecles. The osteons, which are perfectly similar 

 to those in Astraspis (Bryant, 1936, pi. 6, pi. 7, fig. 1 ; 0rvig, 1951, pi. 3, 

 figs. 3, 4) and to those in various Psammosteids as well (Gross, 1954, 

 pi. 5, fig. 11), are penetrated by fine fibers of Sharpey radiating from 

 the vascular canals. 



The basal layer was somewhat thin in early ontogenetic stages, but 

 subsequently it increased considerably in thickness by the apposition 

 of new aspidin lamellae on its inner side. As far as one can judge, it 

 is not continuous basally with layers of perichondral bone. As in 

 Astraspis and Eriptychius (fig. 5, 6; see Bryant, 1936, pp. 421, 425, 

 pi. 2, fig. 3; pi. 13, fig. 1), but contrary to the case in all Silurian and 

 Devonian Heterostraci, it is penetrated tliroughout its extent by a 

 great many vascular canals, ascending with about equal interspaces 

 from the subcutaneous (subaponeurotic) vascular plexus beneath the 

 exoskeleton. The presence of this system of ascending vascular 

 canals, whose more or less funnel-like openings are spread all over the 

 basal face of the plates and scales (pi. 1, fig. 5; Bryant, 1936, pi. 5, 

 fig. 2), is no doubt a primitive condition (0rvig, in MS., a). 



It is worthy of mention that the Pycnaspis plates from the Winnipeg 

 formation of the Williston Basin in Montana and the Whitewood 

 formation in South Dakota referred to above (p. 8) exhibit stout, 

 mushroom-like tubercles which in shape and microstructure are 

 similar to those belonging to mature stages of growth in Pycnaspis 

 splendens, new genus, new species (pi. 2, figs. 1-3). These plates are 

 undoubtedly referable to Pycnaspis, new genus, but for the time being, 

 at least, they cannot be determined as to species. 



