. The Geology of Minnesota . cuate or spirally-enrolled; but in the Nautiloidea the sutures, or lines of junction of the septa withthe walls of the shell or conch, are, as a rule, simple, without abrupt curvature andvery seldom with sharp angles, while in the Ammonoidea these sutures are usuallyhighly angulated or zigzagged; in the nautiloids the sipho is very variable inposition, may be small and cylindrical, but is often large and its walls much thick-ened, while in the ammonoids the sipho is always cylindrical, always marginal inposition and without a thickening of the walls. In the faunas
. The Geology of Minnesota . cuate or spirally-enrolled; but in the Nautiloidea the sutures, or lines of junction of the septa withthe walls of the shell or conch, are, as a rule, simple, without abrupt curvature andvery seldom with sharp angles, while in the Ammonoidea these sutures are usuallyhighly angulated or zigzagged; in the nautiloids the sipho is very variable inposition, may be small and cylindrical, but is often large and its walls much thick-ened, while in the ammonoids the sipho is always cylindrical, always marginal inposition and without a thickening of the walls. In the faunas of the Lower Silurian no representatives of the dibranchiates or ofthe ammonoid tetrabranchiates are known. We have, therefore, to deal in thischapter only with the nautiloid tetrabranchiates. As an illustration of the general structure of these bodies and to show therelation of the animal to the various parts of the shell, we have here inserted adrawing of Nautilus pompilius, reproduced from the well known figure by Richard. Fig. 1. —Nautilus pompilius. a. Mantle. n. Hood. b. Its dorsal fold. 0. Exterior dlgitatlons e. Nidamental gland. P- Tentacles. </? Shell muscle. s. Eye. i. Sipho. X. Septa. k. Funnel. z. Body chamber. CEPHALOPODA. 763 General characters of Owen. It represents the shell as sawn horizontally through its center or along theplane in which it is coiled, with the entire animal lying in the body-chamber; showsthe air-chambers, septa and sipho, and the various external parts of the this is a coiled shell, its structural characters are not different from manyof the forms here discussed in which this shell is straight or but slightly curved. General Characters of the Lower Silurian Cephalopods here Described. We have observed that the Cephalopoda met with in the early Silurian faunasare mainly of primitive types of structure. Their predecessors existed in faunasbefore the Silurian but their remains are of infrequent occurrence, and
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