MICROPETALON PURPUREUM. 63 
The tissue of the intestines is generally highly pigmented and smooth, 
but sometimes contracted with peculiar regularity into surface folds, as seen 
in Pl. 10, fig. 9. The rectum (PI. 10, fig. 70), in all the specimens examined was 
contracted lengthwise in long irregular creases. 
Satisfactory figures of the nervous and water-vascular systems (Pl. 10, 
fig. 7), were not obtained, though the ampullae appear to be bilobed, twice as 
long as wide, and pigmented (PI. 10, fig. 8). 
The reproductive organs (PI. 8, fig. 2; Pl. 10, figs. 7, 2), are lacking in the 
posterior interambulacrum, but show no special peculiarity in their structure. 
Micropetalon purpureum A. Acassiz and H. L. Ciark. 
Plates 29-31. 
Unfortunately there is only a single specimen of this species which, although 
17 mm. in length may not be fully adult. The general aspect and some of the 
details are those of Echinonéus. The broad ovoid ambitus, large triangular 
oral, and oblique anal apertures, recall the specimen of Echinonéus, 16 mm. 
long, from Florida (PI. 5, fig. 17). 
On comparison with a young Echinonéus, 4.40 mm. long, (Pl. 3, figs. 1, 2, 
Pl. 13, fig. 7, Pl. 14, figs. 7, 2, Pl. 15, fig. 7), from Makemo Island, I find the test 
frail in both, the ambulacra flattened, and the breadth of the interambulacra 
at the ambitus in the bivium broader than in the trivium. In Micropetalon 
the adoral end of the anal opening inclines towards the left posterior ambula- 
crum, but this may be an individual peculiarity. The size and grouping of 
the plates on the anal plastron are precisely similar to the same in the young 
Echinonéus, but being further advanced in age, the anal system has acquired 
on each side, two additional interradial plates. The oral membrane in each is 
overlaid with minute caleareous plates, radiating towards the centre. Of the 
genital plates, three are of about the same size, while the left anterior is smaller. 
In Micropetalon the genital pores are present, which is not the case in the young 
Echinonéus. The ocular plates in the bivium are in contact in Micropetalon 
exactly as in the Echinonéus from Makemo. 
The primary tubercles are comparatively few in number, and well separated, 
being grouped, beginning at about the fourth plate from the actinal system, in 
two horizontal rows on each plate, or diagonally over the test; each of the last 
four or more of the abactinal interambulacral plates has only one of these tubercles. 
Like the young Echinonéus (Pl. 24, figs. 1, 2), Micropetalon has only three 
series of primary tubercles in the odd anterior ambulacrum. The first tubercle 
