On THE STRUCTURE AND CLASSIFICATION OF THE. TrEMATASPIDAE. 
19 
be confounded with irregulär projections due to lateral pressure and a downward tlirust of 
fragments of the inner layer. 
The processes just described appear from tlieir position and trend to bave served for 
the attacbment of muscles that were directed sharply downwards and forwards, to some mo- 
vable organs on the anterior ventral margins of the head. 
I know no other Ostracoderms, in wliicli structures comparable with these processes 
hâve been described. In a good specimen of Pteraspis , that was kindly loaned me by Pro¬ 
fessor Sollas from the Oxford collections, I hâve found two semicircular furrows bordered by 
crests that correspond very well with those in Tremataspis. They lie on the outer surface 
of the dorsal shield close to the median line, about opposite the peculiar marginal openings 
(gill openings of Lankester). Transverse sections alone will enable us to demonstrate with 
certainty the homology of these structures in Pteraspis with those of Tremataspis. 
In Limulus , there are two rows of separate infoldings of the dorsal shield that serve 
for the attachment of muscles. If they fused to form continuons plates, they would corre¬ 
spond well with the eutapophyses of Tremataspis. 
In Limulus (Patten and Redenbaugh’ 99), there are two sets of muscles that pass from 
the eutapophyses downwards and forwards, or downwards and backwards. One set goes to 
the haemal, and to the posterior side of the plastron (Plastro, and mesoplastro-entapophysal, 
and longitudinal abdominal) and the other, to the abdominal appendages (abductor muscles 
of gills and operculum), 
The entapophyses of Tremataspis probably gave attachment to a somewhat similar set 
of muscles, that is muscles attached to the walls of an endocranium and to movable appen¬ 
dages, either respiratory or locomotor. 
There are other muscles in Limulus that arise from the inner surface of the cephalic 
shield and pass downwards, some to the plastron (plastro-tergals), others (сохо -tergals) to 
the coxae of the thoracic appendages. The attachment of the сохо -tergal muscles of the five 
pairs of walking appendages to the inner surface of the dorsal shield produces five well de- 
fined pairs of radiating bands or ridges, that form conspicuous features on the outer, as well 
as the inner, surface of the cepbalic shield. The two sets of markings produced by the mus¬ 
cles belonging to the cheliceral and chelarial segments, i. e. the first and last ones of the 
sériés, are much less prominent. I hâve already pointed out, in an address before the Y. 
International congress of Biologists in Berlin, that these ridges in Limulus may be compared 
with the radiating ridges on the inner surface of the dorsal shield of Pteraspis (see Lan- 
kester’s fig. 1, PI. IV) and with the well known sériés of ridges on the outer surface of the 
dorsal shield of Gyathaspis. 
These muscle ridges in Pteraspis and Gyathaspis furnish still further evidence of the 
presence in the Ostracoderms of numerous pairs of cephalic appendages. 
The So Called Endolymphatic Ducts: This well known pair of small circular openings 
in the dorsal shield hâve been regarded as the mouths of endolymphatic ducts by Rohon 
3 * 
