a I 
NEWTON: ON A NEW CTENOSTREON. 24 
C. pectiniformis, Schlotheim, has from 11 to 14. 
C. distans, Eichwald, has from 7 to 8. 
C. hector, Orbigny, has about 18. 
C. Wrighti, Bayle, has about 10. 
It will thus be observed that the Russian form C. distans approxi- 
mates more closely to the Swiss species in the number of its costal 
rays, but in other characters it is different, and moreover shows no 
tubular prolongations or any evidence of the fistulosity of the ribs. 
The spines, which mark the different periods of growth in the life 
of the mollusc, are equidistant, long, tubular, sometimes bulbous, 
tapering, or of nearly equal diameter throughout, whilst the basal 
commencement of each develops as a cylindrical fold from the shell- 
margin, as in Spondylus, and forms a median groove. The spines are 
also of large diameter, frequently measuring 15 millimetres, thickly 
walled, sometimes compressed, often flexuous and curving over the 
ventral margin, where they meet those of the opposing valve; their 
surfaces are ornamented with fine concentric striations. 
This species is represented by a left valve (Pl. V, Fig. 1) exhibiting 
a handsome test showing the bold convex coste with their tubular 
prolongations, one of which measures 70 mm. in length, although with 
an incomplete end; the spinous processes of the opposing valve are 
also seen in the ventral region, otherwise that valve is mostly hidden 
by matrix. The margins of the anterior auricles are much thickened 
and rolled back, as is the case in all Ctenostreons that I have examined, 
but the details of the byssal orifice are not clearly defined. 
The second specimen of importance is the right valve of a larger 
individual than the former, showing two prominent spines on the 
posterior side of the shell, one being compressed and with a tendency 
to a closed end. The other spines are mostly detached at their bases, 
thus giving the fistulose character to the ribs. This specimen also 
shows the bold lamellose structure of the shell; no opposing valve can 
be traced. The median groove and folded character of the spines as 
they arise from the test is well seen in a fragmentary valve Pl. V, 
Fig. 2) of another shell. Further examples, which are otherwise of 
imperfect preservation, exhibit the widely scalloped margins of the 
valves produced by the elevated ribs and the concave interstitial spaces. 
In conclusion, I have to thank Dr. Ernst Sauerbeck for some 
interesting notes and drawings accompanying the specimens collected 
by himself and Professor Burckhardt ; these, although not here 
incorporated, were valuable for reference. A word of praise is also 
due to Mr. H. R. Holder for the excellent photographs from which 
the illustrations have been prepared. 
EXPLANATION OF PLATE V. 
Fic. 1.—Ctenostreon Burckhardti, n.sp. A left valve exhibiting the radial coste and 
their tubular spinous prolongations. One-half nat. size. 
Fia. 2.—Portion of internal surface of a valve with tubular spines proceeding from 
the outer margin, showing folded growth and median groove. Nat. size. 
VOL. V.—ocTOBER, 1902. 17 
