26 BRITISH PALEOZOIC ASTEROZOA. 



stout and closely touching, in Lapwortlinra tliey are tliin and merely overlap at 

 their edges. It is obvious that the adambulacral longitudinal muscles must be of 

 small importance or perhaps absent. Further, the surfaces of contact of the 

 ambulacralia and adambulacralia are as much reduced in Lapworfhnra as in modern 

 Ophiuroids and the form possesses stout Ophiuroid spines. 



The skin on the apical surface must have had fen^ calcifications, for usually 

 the ambulacralia are fully exposed in dorsal view. Observed from this aspect the 

 ossicles, as Sollas remarks (71, p. 217), resemble "the phalanges of a Plesiosaur." 

 Usually there is a large concavity Avliich dips downwards between opposing- 

 ambulacral halves. Somewhat similar median excavations of the ambulacralia 

 occur in Onychaster, which Miss Sollas has suggested lodged diverticula of the 

 coelom (72, p. 52). The point is important, as according to the interpretation of 

 Schondorf there is no dorsal coelom in the Auluroidea, in which group he places 

 LapiDorthnra. A well-marked " ophiuroid " ridge which liears spines can be seen 

 from this view (PI. I, fig. 9). 



B. THE MOUTH PARTS. 



The mouth-frames of Recent Asteroidea and Ophiuroidea are very different in 

 outward appearance, and even now very diverse views are held as to the homo- 

 logies between the various parts. It will be seen that the palgeontological evidence 

 is of great value in helping us to a definite decision on these conflicting opinions. 



The Mouth-frame of Recent Asteroidea. 



The mouth-frame of the Recent Asteroidea retains on the whole a primitive 

 form, and its component ossicles are not diSicult to recognise. If Ave examine the 

 mouth-region in oral aspect (Text-fig. 3), five pairs of prominent projections are 

 seen in the interradial angles bordering the mouth. These projections, when exa- 

 mined more closely, are observed to be composed of pairs of triangular ossicles- 

 united by muscles. They are known as " mouth-angle " plates. Radially the 

 mouth is seen to be bounded by five pairs of ambulacralia. These first ambulacralia 

 are larger than any other ambulacralia, and each possesses two processes instead 

 of the one usually fomid. Between the first two processes rests the first tube-foot. 



A better view of the mouth-parts may be obtained after removal of the apical 

 plates of the centre of the disc and a consequent exposure of the ossicles from 

 above (PI. I, fig. 1). The large first ambulacralia are seen to have a pronounced 

 downward slope towards the centre of the mouth-aperture. This slope is also 

 found in fossil Asterozoa. 



The anterior process of the first ambulacral fits on the apical surface of a 

 mouth-angle plate. Immediately proximal to the process is a groove also found 



