56 Mr. H. J. Carter on the close Relationship of 



10-12ths inch (fig. 13). From this uninliltrated Dortion, then, 

 the structure of Pnrkcria will be chiefly describea. 



It is desirable to premise that the fossilized tissue-fibre 

 averages about 1- 900th inch in diameter, one third of which 

 belongs to the core or central portion, and the rest to the 

 incrustation (fig. 14, «, h). In the stems of Chitina ericopstSj 

 where the fibre is largest, it measures, ichen 7'ound, about 

 1 -900th inch in diameter ; but, of course, this varies slightly 

 in each instance with position ; also it must be premised that the 

 structure of Parkeriaj which is concentric, will be divided into 

 laminte, intervals and vertical tubes, and that the two latter 

 increase in size outwards, so that, while the first interval 

 and tube are respectively l-300th and l-200th inch, the same, 

 five rows from the centre, are respectively l-200th and l-60th 

 inch in transverse diameter. 



Commencing immediately round about the centre, whose 

 structure itself will be more advantageously considered here- 

 after, the first lamina may be observed, under the microscope, 

 in the vertical section, to be composed of two layers of reti- 

 culated tissue presenting between them a line of openings, and 

 to be about 1 -300th incli in thickness ; after this, on progress- 

 ing outwards, the thickness is increased a little, rather by 

 the addition of more tissue-fibre than by the enlargement of 

 the openings. These are the openings of " passages running 

 at right angles to the plane of section," which Dr. Carpenter 

 (to whose faithful descriptions and illustrations in the ' Philoso- 

 phical Transactions,' Z. c, I shall often have to refer) likens 

 to " communications between the contiguous series of cham- 

 berlets in AheoUna " {ojy. et I. c. p. 730) ; but they are more 

 analogous to, if not homologous with, the openings observed 

 in the horizontal lamina of Tuhijjora musica, as will be better 

 understood hereafter. But to return to the thickening of the 

 lamina : in progressing outwards, this may be observed, as 

 before stated, to be chiefly owing to an increase in the amount 

 of tissue-fibre, that, rising into pillar-like forms on the outer 

 surface of the basal lamina, may be seen, in the vertical 

 section, to grow out in tlic same way, on both sides of the 

 succeeding laminae, so that, where meeting their vis-h-vis, they 

 form pillars of support to, and where not meeting remain 

 with free ends in, the interval. 



In the first three or four intervals, this outgrowth of tissue- 

 fibre from the laminai is almost entirely limited to cylindrical 

 pillars scattered irregularly through the intervals, which, when 

 broken, may be observed to be hollow and to extend simply 

 from one lamina to the other (figs. 13, d and 17, Z*). These are 

 the "radial tubes" of Dr. Carpenter. They increase in 



