198 Dr. BiGSBY on the Geography and Geology of Lake Huron. 



Among the specimens that I have seen^ are some which strongly resemble the 

 following species figured in Sowerby's Mineral Conchology : viz. O. flexuosa^ 

 O. Steinhaueri^ O. gigantea^ and O. cordiformis. 



In the niagnesian limestone of Thessalon Island are found many casts of 

 orthocerae of a conical form, with blunt hemispherical terminations and con- 

 cave septa, resembling in their general external outline many of the known 

 species ; but having a siphunculus of a very remarkable structure^ which 

 differs from any thing known respecting this part in any of the chambered 

 shells. PI. XXV. figures I, 2, 3, are drawings of different specimens of this 

 fossil. 



The siphuncle is large, placed laterally, and has an enlargement in each 

 interval between the septa. A tube runs down the axis of the siphuncle, 

 to the sides of which, at the middle of each enlargement, it sends off, at 

 right angles to itself, a number of radii, in verticillations, like spokes from 

 the centre of a wheel. The radii appear to vary in number, and cannot be 

 accurately counted, owing to the state of their preservation, as they are co- 

 vered completely with small crystals of pearlspar. Their number, however, 

 is about 16. In those places whence the radii spring, the tube has enlarged 

 rings : between these rings are elevated longitudinal ridges leading to the radii. 

 The tube diminishes in size toward the lower end of the column ; but does 

 not always taper regularly, nor, in different specimens, to the same degree. 



Small branched corals are common, and I have found retiperae and chain- 

 coral : turbinolias of large size, and well preserved, are abundant. 



Cylinders, slightly conical, perfectly smooth externally, from 1 to 2 inches 

 in diameter at the large end, and from 3 to 18 inches long, are frequently found 

 among the remains of orthocerae. I have not seen their terminations. They 

 consist, within, either of granular limestone, or of confusedly crystallized white 

 calcspar, not radiated as in belemnites. 



Neither any root nor head of an encrinite has been found on Lake Huron, 

 nor, to my knowledge, in America. The columns of encrini are exceedingly 

 numerous in the limestone of St. Joseph and the north ; but they are without 

 lateral arms. 



The Entrochus laevis, annulatus, verrucosus, and prominens of Miller are 

 the most abundant. Their diameter is the same as is usual in English speci- 

 mens, and their length from 8 to 14 inches. Several specimens are covered 

 with a coralline net-work. 



Pentacrinites are common, but very small. The bivalves have not received 

 attention : terebratulas are met with. Among the univalves, euomphali and 

 cirri have been found. 



