C^tiOCODtLIANS, LIZARDS, AND SNAKES. 407 



of both lateral and mediau lobes the plicit* iuosculate, formiug a tripe- 

 like structure. 



Species. — I distinguish eleven species of this geuus, which differ as 

 follows : 



I. Olio series of inargiual abdominal scales. 



a. No cularged gular scales. 



h. Nostrils in line with canthi rostrales. 



Horus reduced to tubercles, the temporals larger than occipitals; intra- 

 labials and enlarged dorsal scales not prominent; ventrals smooth. 



P. (longlassii Bell. 

 Horns short, occi]iitals longer than temporals in the same plane; infra- 

 labials not prominent; large dorsals not prominent; ventrals smooth. 



P. orbicidare Wiegmann. 



Horns short, occipitals longest, directed upward and not in plane of 



temporals; infralabials not prominent; large dorsals prominent. 



P. boucardii Bocourt. 

 aa. Enlarged gular scales jircseut. 

 b. Nares within canthi rostrales. 



A single large temporal horu on each side ; occipitals rudimental; ven- 

 tral scales keeled ; tail very short P. tanrxs Dugcs. 



Three temporals and the occipital subequal ; ventral scales keeled; tail 



very short /'. braconnierii liocourt. 



bb. Nares on line of canthi rostrales. 



Occipitals longest; temporals prominent; the anterior below the orbit; 

 ventrals smooth; enlarged dorsals prominent; enlarged gulars in sev- 

 eral rows; a subrictal spine; two rows of marginals on the tail. 



/'. ccrroense Stejneger. 



II. Two marginal abdominal fringes of elongate scales. 



a. ICiilarged gulars present; enlarged dorsals prominent. 

 b. Nares within canthi rostrales. 



Ventral scales smooth ; four equal elongate occipital horns, forming with 

 the temporals an uninterrupted series; inferior marginal abdomi- 

 nals feeble ; one row of enlarged gulars /'. solare Gray. 



Ventral scales smooth; posterior temporal and occipital turned out- 

 ward; anterior temporal below eye; a subrictal not followed by a 

 spine; several rows of enlarged gulars; a median occipital spine. 



P. coronatum Blainville. 

 Ventrals generally keeled ; occipitals directed upward, the median 

 minut-e; temporal row posterior only; uo subrictal; one row of en- 

 larged gulars . /'. cornutum Harlan. 



bb. Nares on line of canthi rostrales. 



Ventrals smooth ; occipitals and last temporals subeciual, straightj tem- 

 jioral row, but no horn under eye; mediau occipital minute;^a sub- 

 rictal followed by a spine; several rows of enlarged gulars. 



P. blainvillei Gray. 



Ventrals keeled; superciliaries, last temporals, and occipitals sube(|ual, 



the last directed ujjward; no median occipital; temporal row not 



extending below orbit ; no subrictal; several rows of enlarged gulars ; 



enlarged dorsals jiToniiuent at the sides of the back only.. P. asio Cope. 



These si)ecies may be also grouped as follows, by consideration of 

 certain characters of the sqnamation : 



I. Several subrictal projecting plates; temporal row of conic plates not continued 

 to bplow orbit. I^argc dorsal scales without rosette at l)ase. Two occipital 

 horns. Femoral pores in the interior of scuta. 



P. orbicularc; P. doiitjlassii; P. boucardii; P. braconnierii. 



