OCR Text |
Show 182 MR. W. K. PARKER ON STEATORNIS CARIPENSIS. [Apr. 2, Steatornis; and the shape and strength of its leg-bones are in great contrast to what we see in such Fowl-like Coccygomorphae as the Musophagidae and the Ground-Cuckoos. The femur (/.) of Steatornis is, perhaps, the straightest to be seen in the Class ; the breadth is 8*3 millim. above and below, and it narrows to a diameter of 3*5 millim. in the middle of its shaft. The condyloid ledge for the feeble fibula (fb.) is not well-marked; that bone is a little above half the length of the tibia, which latter bone is as straight and primordial as the femur ; it would seem as if these bones had not altered in shape since the middle of incubation. The breadth of the head of the tibia (t.) is 9 millim., across its tarsal base 8 millim., whilst the middle of the shaft is only 3 millim. The cnemial ridges are very rudimentary, and the ridge outside for the fibula (fb.) only reaches 15 millim. downwards. A thin delicate tendon-bridge exists in front of the base of the tibia below (Plate X X . fig. 4 ) ; but there is no special depression between the astragalar and calcaneal regions of the condyle, for the intercondyloid knob on the distal tarsal (or head of the shank) is nearly obsolete (Plate X X . fig. 3). The inner part of the condyle is formed by the astragalus, and the outer by the os calcis; there is a rudiment of the intermedium between them ; the centrale, or "naviculare" (nv.), is seen as a cartilaginous, curved wedge behind the joint. The tarsal outgrowth behind the head of the shank to form a tendon-canal for the plantar tendons (Plate X X . figs. 3, 5) is open. This part is closed in, and forms one canal in Ceryle alcyon; in the Martin (Chelidon urbica), as in all the Coracomorphae, there are five canals in the compound mass ; in the Swift (Cypselus apus) there is an unusual thing-a little bridge in front of the distal tarsal; but the two ridges behind are not united1. There is a notable peculiarity in the structure of the inter-tarsal joint. The condyloid trochlea formed by the astragalus is large and perfect, and rolls in a well-formed concavity on the inner side of the great distal tarsal. But the calcaneal part of the double trochlea is feeble (Plate X X . figs. 3-5) and the outer part of the facet on the lower tarsal is almost flat2. The tarso-metatarsus shows the signs of division into three main metatarsals, both above and below, 2nd to 4th (Plate XIX. figs. 4, 5). The free distal piece that carries the "hallux" (mt.t'.) is 5*5 millim. long. On the outer side of the head of this small shank the 5th metatarsal (mt.t. 5) can be seen as a club-shaped rudiment, fused with 1 The classification of birds by the palate is \ery useful as a help to otber methods, everything else being taken into account. Nowhere does it show its value more than in the Coracomorphfe; they are all " .ZEgithognathous " ; but the iEgithognatha*; and the Coracomcrpha*; are not equal groups-the former is too large for accurate superposition on the latter; the. Swifts (Cypselidae) are iEgithognathiB, but they are not Ooracomorphge. 2 This obliquity reminds one of that in the free astragalus itself in the Megatherium, as compared with the same bone in the Horse, the latter having the condyle in two nearly equal, oblique semicircular elevations, whilst in the former the two convexities are extremely unequal. |