56 
PALEONTOLOGY: R. RUEDEMANN 
Proc. N. a. S. 
like basal tubes that served as anchors, with distal or upper extremities, 
confused masses of long, rigid or flexed thecae giving the part the ap- 
pearance of a Medusa-head. By frequent division into bundles of thinner 
tubes, these thecae sometimes produce a still denser mass, in which the 
axis is a compound, spirally rising platform composed of broader thecae. 
In this respect the structure is identical in appearance with the graptolite 
genus Cactograptus. 
Worms abound in certain beds of both the formations named, several 
entire individuals with distinct segmentation having been found, one 
clearly referable to Protoscolex. 
Among the brachiopods a species of Lingula preserves the original 
concentric color-bands. 
Most interesting among the cephalopods is a Hexameroceras, in which 
the sides of the slit leading to the hyponomic sinus are provided with inter- 
locking teeth, serving for protection, as in the case of the narrow slit-like 
aperture of the living Cypraea and other gastropods. This and other 
observations made upon these Silurian cephalopods are significant as sup- 
porting the view lately advanced by Prell {Centralblatt, No. 10, 1921, pp. 
303-315) that in the early nautiloids of the Phragmoceras and Gompho- 
ceras types with variable and contracted apertures, the animals lacked 
all cartilaginous cephalic parts and were so plastic that they were able 
to squeeze their entire cephalopodium out; thus comparable to the recent 
Helix and Cypraea. The apertural sinuses which have served to dis- 
tinguish the so-called genera Pentameroceras, Hexameroceras, etc., 
are therefore not as constant features as has been supposed, nor did they 
serve as exits for arms. 
