O') 
s. s. BUCKMAN. 
(Schwed. Südpolar-Exp. 
form loop, and that certain Tcrcbratcllœ are really Magellaniœ which have not yet 
lost the terebratelliform loop. This means that a rearrangement of these genera to 
correspond with vertical lines of descent instead of to indicate horizontal lines of 
developmental stages may be anticipated. 
It is known that among terebratuloid forms the shape (test and ornament) may 
change considerably while the loop remains without much modification; and on the 
other hand that the loop may change considerably while the shape remains little 
altered. It may be suggested then that the likeness of Magcllania vcnosa to forms 
of PacJiymagas which has led to their confusion is really explainable on the idea 
that while the shape of Pachymagas has remained from generation to generation 
little altered, the loop has been rapidly undergoing modification at an earlier and 
earlier age (tachygenisis), passing from terebratelliform to magellaniform ; until what 
was an adult loop of Pachymagas becomes the youthful loop of Magcllania vcnosa^ 
while the adult shape of Pachymagas, being unaltered, becomes both the youthful 
and adult shape of ]\I. vcnosa. 
Locality: Cockburn Island (13), off Graham Land, Antarctica. 
Formation: Glauconitic Bank. 
Material: Seven more or less fragmentary shells. Six of these are only ventral 
valves or portions of such valve; only one has both valves, and that is a very much 
crushed and distorted example. 
Position of kindred species: Tcrcbratclla venter, Jhering, belongs to the Upper, 
Middle and Lower Patagonian formation (Jhering), and Tercbratella gigantca, Ort- 
MANN, to the Cape Fairweather beds, which are later than Patagonian (Ortmann). 
PiLSBRY referring to a collection of Tertiary fossils from Cape Fairweather (Pa- 
tagonian Tert. Foss. 329) considers that they indicate Pliocene, and he remarks that 
Magcllania venosa, SOLANDER, is abundant. But according to Ortmann (Pat. 
Exped.; Pal. 78) this identification is incorrect, and he names the species Terebra- 
tel la gigant ea. 
Genus Magcllania, Bayle. 
Magellania fontainei, d’Orbigny sp. 
Plate III, fig. 6. 
1847. Tcrebratnla fontainei, d’Orbigny, Voy. Amér., V, 675; T. Fontaincana, d’Orb. 
Id., PI. 85, fig. 30, 31 (reduced. Vs)- 
i886. Waldheimia Fojitaineajia ; DavidSON, Recent Brach., PI. VIII, fig- 6 (legend of 
Plate); W. venosa; Id., p. 49. 
