URASTERELLA. 145 



The adambulacralia are better seen in apical view in the majority of these 

 specimens than is usual. This is because the ambulacra! groove has been forced 

 widely open with the result that the ambulacralia are thrust well on to the sides 

 of the arm. The dislocation is helped, of course, by the fragile nature of the form. 



Oral Surface (Plate X, fig. 3). — As already mentioned, the groove in four of the 

 specimens is widely open, and the ambulacralia are thus fully exposed. They are 

 of the type already described for U. graijse. Each is surmounted by a stout 



!-shaped ridge, the long arm of the ! being set somewhat obliquely to the main 



body of the ossicle. Good views of the ambulacralia are seen in the photograph 

 (PI. X, fig. 3). The inner faces of the adambulacralia, which in the natural 

 position would form a steep wall to the groove, are crushed outwards so that they 

 lie flush with the ridges of the ambulacralia. This position makes the true oral 

 ridged faces curve proximalwards exactly as we shall see they do in Sturtzaster 

 (see below). The mouth-angle plates are so small that they can only be recognised 

 with difficulty. The first ambulacralia, on the other hand, are very stout and form 

 a closed ring round the mouth. Each possesses a deep central depression for the 

 first tube-foot. 



An odontophor can be seen in each of the interradial angles. 



Casts from mould 70 show the arms somewhat coiled and the groove closed. 

 The ridges on the adambulacralia are especially discernible on this specimen. 



Measurements. — 93 gives R : r : : 10 mm. : 1 mm. The remaining specimens are 

 of about the same size. 



Horizon and Locality. — The Starfish-bed of Gutterford Burn (Wenlockian), 

 Pentland Hills. 



Note. — It is difficult to connect this species with either U. thraivensis or 

 U. ruthveni. At first sight it might seem to be the small beginnings of the 

 U. ruth rent lineage, but the Wenlockian of Gotland (see p. 147) contains a much 

 larger and more typical specimen of that lineage, suggesting that we must look for 

 the roots of the lineage in older strata. 



American Species of Urasterella. 



Urasterella pulchella, Billings. — This species has been fully described by 

 Schuchert (85, pp. 178-180) and Hudson (93, pp. 117-139). It is found in the 

 Trenton (M. Ordovician) formation of New York and Canada. The cross-section 

 of the arm reproduced here (Text-fig. 80) shows that adraclialia of the first row 

 (the supero-marginalia of Schuchert and Hudson) are conspicuously larger than 

 the remaining adradialia, and the species therefore is a true Urasterella. Other 

 details of the structure have already been incorporated in the preceding general 

 account of the structure of the family and genera. The species may attain a large 

 size (R : r : : 52 mm. : 11 mm.). This is larger than any English species. 



