Th. Mortensen : On the alleged primitive Ophiuroid, Ophioteresis elegans Bell. 7 



as a distinct genus, comprising both the species elegans and tigris, must be post- 

 poned till after the results of my study of the anatomicai stucture of the two 

 forms has been set forth. 



The first important point to settle is, of course, the presence or absence 

 of the ventral piates. Lyman says in his description af O. tigris : "under arm- 

 plates covered with thick skin and seen indistinctly, except when dry. They have 

 an irregular triangular form, with a peak within and a lumpy surface. They cover 

 only a part of the arm, and differ in figure one from another". The figure given 

 by Lyman (PI. I. Il) shows only the two inner ventral piates and accordingly gives 

 no clear representation of their form. Bell repeatedly emphasises the total absence 

 of the ventral piates in Ophwteresis, but his fig. 3 shows a series of piates along 

 the ventral side of the arms. As Bell does not say anything of these piates, he 

 has probably taken them to be the vertebræ. In reality the matter lies thus: 

 the alcoholic specimen of Ophioteresis has the ventral surface of the arms covered 

 by a thick, somewhat leathery skin, which completely obscures the piates or the 

 vertebræ. When the specimen is dried, a series of piates are seen (PI. I, Fig. 10), 

 rather like those seen in the figure quoted in Bell's paper. In order to find out 

 whether these piates are real ventral piates or only the ventral side of the vertebræ, 

 they had to be treated with Eau de Javelle. The result is shown in figs. 6 — 7, 

 PI. II ; it is seen from them that there is a large, nearly heartshaped ventral plate, 

 which covers most of the ventral side of the vertebra ; it is only at the sides that 

 the vertebra itseif appears. It must be agreed, however, that the Hmits of the 

 ventral plate are by no means easy to observe, the plate lying very close to the 

 vertebra and having its very thin edges at a level with the vertebra, as is well seen 

 in the endview, PI. II, Fig. 6. But the faet that the plate is loosened from the 

 vertebra by the dissolving fluid, leaves no doubt that it is the real ventral plate. 

 It has a slight depression along the middle line — the ventral furrow of Bell 

 — which has given rise to the behef that there is an open ambulacral furrow. The 

 cxamination of the point of the arm gives a no less deiinite proof that we have 

 here the real ventral piates; a mere glance at Fig. 2, Pi. I, which represents the 

 underside of the point of an arm of Ophioteresis, will leave no doubt that the plutcs 

 lying there in the middle hne are really the ventral piates. On passing farthcr 

 down (orally) along the arm the ventral plates are seen gradually to assumo the 

 same shape as in the fullgrown joints. 



With regard to Ophiothela tigris I find that the ventral plates are arranged in the 

 same way, only their shape is more elongate, and they cover a somewhat smaller part 

 <if the vertebra ; furtber the depression along the median line is hardly so distinct as 

 in O. elegans (PI. I Fig. 9). Tbis difference in the shape of the ventral plaiet 



