6o COXiN'ECTICUT GEOL. AND NAT. HIST. SURVEY. [Bull. 



as their name implies, are a series of small plates surrounding the 

 orbit. They are of polygonal contour (see Fig. 7), and are 

 arranged in much the same fashion as in Lepidotus, those along 

 the inferior border being of large size and extending some dis- 

 tance in advance of the upper row^. Indications of a mucous 

 canal are observable over part of the circumorbital ring in some 

 species. Immediately below these plates are situated the sub- 

 orbitals, which are fewer and much larger than in Lepidotus, 

 Dapedius, and related genera. The line of demarcation between 

 the suborbitals and contiguous plates has not been satisfactorily 

 determined in any species thus far investigated.^ The postorbital 

 is a large thin plate situated between the hindermost circum- 

 orbitals and the operculum. It is sometimes in contact with the 

 last-named plate posteriorly, as in S. bergeri and possibly in 

 5". nilssoni, but may be entirely separated from it by the pre- 

 operculum, as in 5". capensis. 



The opercular apparatus consists of (i) a large operculum, 

 of variable shape, but generally with a narrower upper border; 

 (2) a narrow, falciform preoperculum, with the mucous canal 

 interrupted and appearing as a series of perforations; (3) a 

 suboperculum, the exposed surface of which generally exhibits a 

 sublunate outline; and (4) a triangular interoperculum. The 

 posterior borders of the operculum and interoperculum are em- 

 braced by a large and heavy plate, often very conspictious, the 

 clavicle. This is similar to the preoperculum in form, but is 

 much more solid, and its terminal angle in front is frequently 

 thickened or otherwise prominent. It is succeeded behind by 

 one or two enlarged postclavicular scales. There is a series 

 of branchiostegal rays, but these, like the coracoid, are seldom 



* Nevertheless, the relations of these plates and also other details of the cranial 

 osteology are shown with considerable clearness, amounting almost to certainty as 

 regards some features, in a number of specimens of Semionotus from a locality a 

 few miles north of Guilford, Connecticut. The material referred to was collected 

 nearly a score of years ago by Mr. Loper, and is now preserved in the U. S. National 

 Museum at Washington. Peculiar conditions of weathering, and perhaps also 

 the admixture of much argillaceous matter in the rock, are responsible for the excellent 

 portrayal of details. These specimens suggest an image of what the facial and 

 cranial elements should look like, but the image is blurred, and refuses to shape 

 itself in hard and fast lineaments which are requisite for a dependable restoration. 

 It seems better to resist the temptation to reconstruct the arrangement of head 

 parts from material which is highly suggestive but still not quite decisive. The same 

 applies to well preserved specimens of Catopterus from the Connecticut Valley region, 

 and to equally perfect examples of Perleidus from the Alpine province. 



