20 William Patten. 



(92, p. 69), partly because they were too large for the passage of blood vessels, but princip- 

 ally because there was no other vertebrate organ with which to compare them. nWomit sie 

 soiisf noch mi vergleichen waren ist mir wenigstens unerfindlich) says Rohon. His conclusion 

 was based on an examination, without special preparation, of the openings as seen from with- 

 out. In the Petersburg specimen, that I was allowed to section, one of these openings came 

 near enough to the section plane to show that the structures in question are the openings of 

 small tubular invaginations of the shell. The inner end of the tube appeared to be closed, 

 although whether it was, or not, could not be determined with certainty. 



A blind tube, such as this 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^ 

 pliyses for the attachment of muscles. Other members of the series lying farther back in 

 the same line may have united into the continuous bony ridge that we have already 

 described. It is interesting to observe in this connection that in Limulus the anterior pair 

 of entapophyses are much the largest of the series, and arise independently from the 

 posterior margin ofthe head shield, 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 depressions like dots and dashes, arranged in 

 linear series. 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 

 differences in the number and length of the dots and dashes, due apparently to differences 

 in age, the arrangement of the main lines is, on the whole, remarkably constant. As no 

 one specimen was found perfect enough to show 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 shown, on the right and left sides, the difference between the 

 circum-orbital lines in very young and very old specimens. 



Each marking consists 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, suggesting a chain of 

 bacilli some of which are undergoing transverse fission. 



The bottoms of the grooves appeared perfectly smooth and regular, even wheri 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 regular series 

 of short close set dashes, or the dashes may in older specimens unite to form a continuous 

 furrow. The number and size of the dashes, therefore varies somewhat in the different lines, 



