1883.] 321 [Hyatt. 



ventral saddles. In America we find Torn. (Gon.) Mithrax, sp. 

 Hall, Nat. Hist. N. ¥., Vol. 5, pt. 2, pi. 69, bicostatum, ibid, pi. 72, 

 fig. 8-10, and the type, Torn (Gon.) uniangulare ibid, pi. 71, 72. 



Maeneceras, 1 nobis, includes forms in which the first pair of 

 saddles are broad, somewhat flattened, and during growth tend to 

 become subdivided. The inner angles become subacute, and small 

 marginal lobes appear between the outer angles and the ventral 

 lobes. These changes may take place late in the life of theani- 

 mal or be inherited at comparatively early stages in accordance 

 with the lawof concentration of development in the more compli- 

 cated species. The dorsal sutures change correllatively, and 

 acquire a pair of subacute dorsal saddles on either side of the 

 annular lobe as in Sandbevger's Maen. terebratum, Verst. Nass. 

 pi. 5, fig. 3 d. Sandberger's figure 4 of Maen. bifer, and his fig- 

 ure 5 c of Maen. (Gon.) delphinum show, that as in Sporadoceras 

 the dorsal saddles were connected with a second pair of saddles, 

 and not probably with the piimitive saddles. The larval stages, 

 are identical in form and sutures with Parodiceras. Maen. 

 (Gon.) bifer, sp. Sandb. ibid, pi. 9, fig. 4, is closely allied to Parod. 

 oxycantha but has the genetic characters of this genus in the 

 later larval stages. Maen. (Gon.) terebratum, sp. Sandb. ibid, 

 pi. 5, fig. 3, exhibits all these characteristics, and the changes in 

 the first pair of saddles described above at still earlier stages. 

 The adult form of this species is the Maen. (Gon.) acuto-laterale 

 Sandb., pi. 6, fig. lb. The whorl in this stage is no longer 

 rounded, but subacute, and the first pair of saddles are completely 

 divided by rounded lobes. We have studied the intermediate 

 adolescent stages of this species, and have been able to connect 

 the two figures quoted by a fine specimen precisely intermediate 

 in size and all its characteristics. Coll. Mus. Comp. Zool., type 

 Maen. acuto-laterale, sp. Sandb. 



Sporadoceras, 2 nobis, includes species which like Maeneceras 

 have additional saddles developed from the division of the first 

 or primitive pair of saddles, but these are generally pointed in 

 adults and the first and second pairs of lobes hastate. The forms 

 of the shells still remain similar to those of Parodiceras and 

 Maeneceras and the magnosellarian saddles are undivided. A 



1 Mtfvq, a crescent. a Siropa8o's> scattered. 



PROCEEDINGS B. S. N. H. VOL. XXII- 21 FEBEtTARY, 1SS4. 



