BELONOSTOMUS CINCTUS, PROM THE CHALK OF SUSSEX. 147 



passed into a horizontal extent measuring 8 millim. across. The 

 teeth upon this surface are quite mamilliform — somewhat suggestive 

 of the tubercles upon the test of certain Echinoids, like Cidaris ; 

 and some of the stages by which this extreme form is reached are 

 illustrated in figs. 11-13. 



It thus becomes evident that the original specimens described by 

 Agassiz as portions of the mandibular rami of Belonostomus cinctus 

 are really fragments of the presymphysial bone of this species. 



In the closely allied genus, Aspidorhynchus, a relatively small, 

 dentigerous, presymphysial bone has been known for some years, and 

 its homologies have already been discussed, in conjunction with the 

 corresponding element in certain Dinosaurs, by Messrs. DoUo * and 

 Hulke t. It is only lately, however, that the same bone has been 

 discovered in Belonostomus. A specimen of B. speciosus, Wagn., 

 from the Lithographic Stone of Bavaria, described last year by Otto 

 Reis (he. cit.), reveals a presymphysial bone of considerable size, 

 articulating with the mandible by a < - shaped suture ; and a de- 

 tached example of the same bone of the Liassic B. Anningicet, in 

 the British Museum (no. P. 513), exhibits the same peculiarities 

 (fig. 14). In B. cinctus, as described above — and as is especially 

 well shown in the original of fig. 9 — the surface of sutural union 

 with the symphysis is one plane without angulation ; and the pre- 

 symphysial element itself may perhaps be relatively larger than in 

 the earlier species. It may also be added that the mandibular rami 

 of Aspidorhynchus present a very close approximation to those of 

 Belonostomus, although the splenial, in the last-named genus, assumes 

 greater importance as a dentigerous element, and the teeth it sup- 

 ports appear to be more adapted for crushing. 



Some interesting general considerations are suggested by the 

 foregoing brief study of the detached mandible, when taken together 

 with certain other known points in the skeletal anatomy of Belono- 

 stomus ; but these questions have been so recently discussed in the 

 memoir by Otto Eeis already quoted, that it will sufiice in conclu- 

 sion merely to note that he has treated the subject in detail. 



EXPLANATION OF PLATE YII. 



Fig. 1. Crushed head of Squatina Cranei, A. S. Woodw., natural size. Lower 

 Chalk, Clayton, near Brighton : ao, antorbital process of cranium ; 

 ch, portion of ceratohyal ; ex, excavated anterior extremity of cranium; 

 hm, hyomandibular ; md, portion of mandibular ramus ; pla, postero- 

 latei'al angle of skull ; ptq, pterygo-quadrate. 



2. Anterior upper tooth of ditto, side view, twice natural size. 



2 a. Base of another, under view. 



3. Anterior lower tooth of ditto, inner aspect, twice natural size. 



3 a. Base of another, under view. 



* L. Dollo, " Quatrieme Note sur les Dinosauriens de Bernissart," Bull. Mus., 

 Eoy. d'Hist. Nat. Belg. vol. ii. (1883), pp. 226-229. 



t J. W. Hulke, Presidential Address, Proc. Geol. Soc. 1884, pp. 47-51. 



X Since the reading of this paper 1 have discovered that this species is truly 

 referable to B donor hynchus, Bronn, and is not improbably identical with 

 B. acutiis, Agass. sp. 



