6o 
PATAGONIAN EXPEDITIONS I PALAEONTOLOGY. 
all rotundate. Shell Gap, upper horizon ; 7 sp., 3 small, 1 large, are alate, 
the rest rotundate. Lake Pueyrredon, base; 27 sp., 3 of them (1 medium, 
2 large) are alate, the rest is rotundate. 
Distribution: San Julian (Des., v. Ih., Lah.); Port Desire, coll, by Dar- 
win (Des.). Darwin (1846) does not mention a Scutella from Port Desire, 
but mentions two forms (p. 110 and 112) from New Bay and San Julian. 
But the occurrence of this species at Port Desire is confirmed by Lahille. 
Jegua quemada (v. Ih.); Lake Buenos Aires (Lah.). 
v. Ihering gives for Echinarachnius the Patagonian formation, for Scu- 
tella the Suprapatagonian ; but, according to our material, both are found 
associated in the same beds. 
Affinities: The most closely allied form among the recent species is 
Echinarachnius mirabilis (Barn.) from Japan (Agassiz, 1873, p. 526, pi. 
13a, f. 5, 6) ; it resembles closely the rotundate form, but the anus is mar- 
ginal, and the interambulacra are broader. The alate form resembles 
much the Miocene Scutellce of Europe, especially S. subrotunda Lmck. from 
the lower Miocene of Bordeaux (Agassiz, 1841, p. 76, pi. 17). 
The presence of this Scutella in the Patagonian beds points strongly to 
their Neogene age. 
Fam. CA SSID U LI DAS Ag. 
Gen. CYRTOMA McCl. 
5. Cyrtoma posthumum Ortmann. 
PI. XII, Fig. i°> 6 . 
1900 Cyrtoma posthumum Ortmann, in: Amer. Journ. Sci., v. 10, p. 369. 
Test subcircular-elliptic. Apex central. Upper side much depressed, 
only slightly convex, covered with very fine and crowded tubercles. Am- 
bulacra petaloid, open, lanceolate, subequal, extending about two-thirds 
from the apex toward the periphery, the two posterior ones are closer to- 
gether than the others. Pores jugate. Anus situated on the upper sur- 
face, in a deep depression, which commences about half way between the 
apex and the posterior extremity by a narrow slit, widening suddenly to- 
ward the margin and giving the anal depression a pyriform shape. Lower 
surface of the test concave, covered with larger, more distant and some- 
what irregular tubercles, a comparatively smooth band running from the 
