20 
William Patten. 
(92, p. 69), parti y because tliey werc too large for the passage ol blood vessels, but princip- 
ally because there was no otber vertebrate organ with wliich to compare them. « Womit sie 
sonst noch zu vergleichen waren ist mir wenigstens unerfindlich) says Robon. His conclusion 
was based ou an examination, without special préparation, of the openings as seen from witli- 
out. In the Petersburg specimen, that I was allowed to section, one of these openings came 
near enougli to the section plane to show that the structures in question are the openings of 
small tubulär invaginations of the shell. The inner end of the tube appeared to be closed, 
although whether it was, or not, could not be determiued with certainty. 
A blind tube, such as tliis appeared to be, can hardly be compared with the endolymphatic 
duct of a Vertebrate, but it is much like the dorsal entapophysis of an Arthropod like Li- 
mulus. I propose, therefore for the present to regard these tubes as ingrowths or entapo- 
physes for the attachment of muscles. Other members of the séries lying farther back in 
the same line may have uuited into the continuous bony ridge that we have already 
described. It is interesting to observe in tliis connection that in Limulus the anterior pair 
of entapophyses are much the largest of the séries, and arise independently Irom the 
posterior margin of the liead sliield, while the remaining six pairs belong to the abdominal shield. 
The Lateral Line Organs: A careful examination of the dorsal surface of Tremataspis 
reveals the presence of numerous shallow dépréssions like dots and dashes, arranged in 
linear séries. These markings are easily seen in all specimens where the smooth outer sur¬ 
face of the shell is well preserved, and while each individual shows certain characteristic 
différences in the number and length of the dots and dashes, due apparently to différences 
in âge, the arrangement of the main lines is, on the whole, remarkably constant. As no 
one specimen was found perfect enougli to sliow all the different lines, to avoid multipli¬ 
cation of figures, I have introduced several drawings made from different specimens into one 
figure (PI. II. fig. 9). 
I have tried, so far as possible, to use for this purpose heads of the same age, except 
where I have purposely sliown, on the right and left sides, the différence between the 
circum-orbital lines in very young and very old specimens. 
Each marking consiste of a short groove from .05 or less to about 1.00 m. m. long, 
either quite straight or a little bent or wavy. The adjacent dashes in the same line may 
be either short or long, or the short and long dashes may alternate, suggestiug a chain of 
bacilli some of which are undergoing transverse fission. 
The bottoms of the grooves appeared perfectly smooth and regulär, eveu wlien seen 
under a moderately high power (x 100). It therefore does not seem probable that the 
organs, if any, situated in these grooves could have received their nerve supply through the 
shell directly from below. Some lines, as the circum-orbital, may consist of a regulär séries 
of short close set dashes, or the dashes may in older specimens unité to form a continuous 
furrow. The number and size of the dashes, therefore varies somewhat in the different lines, 
